Professor Janet Mertz

Born 1949

Mertz was pivotal to the discovery of the first enzyme for easily joining together DNA from different species and designing the protocol that underpinned the development of the first recombinant DNA cloned in bacteria. Her work not only helped lay the foundation for the development of genetic engineering, but also spurred on the establishment of the first safety guidelines for laboratories involved in genetic manipulation. She has also made key contributions to our understanding about how the human tumour viruses SV40, hepatitis B virus, and Epstein-Barr virus regulate expression of their genes and identified roles oestrogen-related receptors play in breast cancer and responses to therapies.

Janet Mertz, 1980 (Photo credit: Janet Mertz)


Mertz's parents were the children of Jewish immigrants who came to the US in the early 1900s from Austria-Hungary (now Poland) and Hungary. They were brought up on the lower east side of Manhattan and the Bronx, New York. Her father, Harry, was an electrical engineering graduate from the City College of New York and served in the US Army Signal Corp during World War II. His first employment after the war was with the US Postal Service. Mertz's mother, Pauline, was a chemistry graduate from Hunter College. Following graduation, she was employed as a playground director by the Works Progress Administration during the Great Depression and, subsequently, as a secretary with a New York City high school.

Both of Mertz’s parents later on worked as laboratory specialists in high schools in New York City. Her father specialised in physical sciences and her mother in biology. It was they who encouraged Mertz’s interest in science, plying her with science books to read from a young age. They also regularly borrowed laboratory equipment from their workplaces to help with her annual science fair projects on a wide variety of topics.

Mertz was the younger of two children. Her older brother earned his undergraduate degree in electrical engineering from Cooper Union College in New York. He went on to work as an electrical engineer and computer scientist in industry. Mertz married Jonathan M Kane, a mathematician and computer scientist who worked at the University of Wisconsin at Whitewater. They have two sons, Daniel and Jeremy. Daniel is a mathematician and computer scientist on the faculty at the University of California at San Diego. Jeremy is a Master-level chess player who helps manage a company that provides after school classes in chess and computers.


Mertz attended the Bronx High School of Science, a science specialty high school in New York City. Ranked among the top 50 schools in the US, the school gave Mertz opportunities to take advanced mathematics and science courses and to try doing some research. She also participated in numerous advanced science courses run on Saturdays at Columbia University. These courses, sponsored by the US National Science Foundation, targeted very bright high school students. It was through one of these courses that Mertz first came across James Watson’s book, the Molecular Biology of Genes. Reading it soon after it was first published in 1965, she quickly became hooked on the idea of becoming a molecular biologist working at the forefront of discovering how life worked at the basic chemical level.

Following school, Mertz attended the Massachusetts Institute of Technology (MIT). Mertz chose to study at MIT in part because it was one of the few top-ranked science-intensive US universities at the time that admitted women. During her degree, Mertz took several courses on bacterial and animal viruses. Two of her courses were taught by David Baltimore who went on to win the Nobel Prize in 1975 for co-discovering reverse transcriptase, an enzyme responsible for the production of complementary DNA from an RNA template. Mertz learned directly from Baltimore about the experiments he performed that led to this discovery. In addition to Baltimore, Mertz gained expertise in the genetics of bacteria and viruses that affect them (bacteriophages) by working in the laboratories of Ethan Signer and the 1969 Nobel Prize winner Salvador Luria. Mertz completed her undergraduate degree at MIT in 1970, having majored in life sciences and electrical engineering.

Through her studies at MIT, Mertz gained a strong desire to perform research on viruses that infect human cells. She viewed such studies as a route to understanding how gene expression is regulated in humans and how such viruses disrupt this regulation to induce cancers. Following the advice of her MIT adviser, Harvey Lodish, she applied to several top-ranked US biochemistry departments for her graduate studies.

In September 1970, Mertz joined the biochemistry department at Stanford University. At this time, the department was quite small. It had just six beginner graduate students who could choose from ten faculty as their supervisors. After talking to several of the faculty, Mertz settled on doing her doctoral research under the mentorship of Paul Berg, who was just beginning his pioneering work related to gene cloning techniques. He invited Mertz to work with him on figuring out how to replicate and express recombinant DNAs in Escherichia coli (E. coli), bacteria that live in the guts of animals including humans. Berg saw this as a means to develop the tools needed to understand gene expression in human cells and its misregulation in cancer, an area close to Mertz’s own interests.

Mertz carefully weighed the pros and cons of the research project suggested to her by Berg. On the one hand, if successful, it promised major advances for genetic engineering and the possibility of developing gene therapy. Yet, she was concerned about its potential implications for eugenics. She was keenly aware of this issue because of her grandparents’ East European Jewish background and the Nazi experiments during the Second World War. In the end, she rationalised that cloning recombinant DNA in bacteria was sufficiently far removed from altering genes in humans.

The project Berg set for Mertz was one he had outlined in his grant application for the American Cancer Society in the autumn of 1970. His aim was to construct a recombinant DNA consisting of DNA from three sources. The first was the entire genome of the simian virus 40 (SV40), a primate virus. The second was a region of DNA from the bacteriophage ? that contains the genes and DNA sequence needed for its DNA to replicate in E. coli. The third was a piece of DNA from E. coli which encodes all of the genes needed to metabolise galactose (gal), a simple sugar. Once made, this recombinant DNA was to be inserted into E. coli so it could be replicated to produce large amounts of it. Mertz hoped to employ this method to be able to grow mutants of SV40 that might not reproduce in primate cells for use in her cancer research studies. Berg also wanted to insert this recombinant DNA into mammalian cells to see if it would either replicate or express the genes needed to metabolise gal.

By the late spring of 1971, Mertz had gained sufficient skills and experimental tools from colleagues in Berg’s laboratory and the neighbouring laboratory of Dale Kaiser, which was also developing genetic engineering techniques, to launch her cloning experiment. In collaboration with Douglas Berg, a postdoctoral fellow in the Kaiser laboratory, she had already achieved two of the key steps. First, she had isolated a circular DNA molecule, the plasmid dvgal120, which contained the desired genes from both the bacteriophage and E. coli. Mertz had also demonstrated that she could purify this chimeric plasmid DNA in the laboratory, and then re-establish it in E. coli employing a method developed in the Kaiser laboratory for use with linear bacteriophage DNAs. Thus, she was well on her way toward cloning recombinant DNA in bacteria. She was just waiting for David Jackson, a postdoctoral fellow in Berg’s lab, to successfully develop a method for joining together the DNA from dvgal120 and SV40 in a test tube.

Mertz's plans, however, were to change after June 1971 following her attendance at a course led by Robert Pollack at the Cold Spring Harbor Laboratory (CSHL) to learn some more techniques for working with animal cells and viruses. On hearing about her intended cloning experiment, Pollack immediately raised concerns about its safety. He was especially worried about the insertion of genes from SV40 into E. coli. SV40 is a fairly harmless virus that mostly lives in some monkey species. However, it also frequently infects humans and can induce the formation of tumours in rodents and human cells in culture. Thus, although SV40 was not known to cause any diseases in humans, Pollack feared that SV40-containing bacteria could escape from the laboratory into the environment, infecting people and other mammals, possibly giving them cancer.

Mertz considered the possible risk raised by Pollack to be quite minimal, especially if one used a strain of E. coli unable to survive outside the laboratory. Nonetheless, she quickly decided against proceeding with her cloning experiment because she realised that it would be unwise for her to be at the centre of a major controversy as a young graduate student. Furthermore, by the time Jackson had managed to construct the SV40-dvgal120 recombinant DNA she needed for the experiment, in the autumn of 1971, she could not proceed any further. This was because Berg had self-imposed a moratorium against anyone performing genetic engineering experiments in his laboratory that introduced SV40 DNA into E. coli until the potential safety concerns had been addressed. Berg had taken this action following conversations with Pollack, Joshua Lederberg (a Nobel prizing-winning geneticist at Stanford University), Maxine Singer (then a section head at the US National Institutes of Health), and multiple other scientists. Thus, even if Mertz had wanted to continue with her planned cloning experiment, she would have not been permitted to do so.

Berg’s policy, however, did not prevent Mertz and the rest of his research team from continuing to work on other projects related to the development of recombinant DNA techniques. As early as 1971, Jackson and Peter Lobban (a graduate student in the Kaiser laboratory) had developed a method for joining together two DNAs. Their method, however, was quite complicated and inefficient. It required the use of six different enzymes.

After dropping her planned cloning experiment, Mertz turned her attention to some other projects she had concurrently been working on. One of these involved examining the infectivity of different forms of SV40 DNA in primate cells. This research led Mertz to discover a very much simpler and more efficient method for joining together DNAs from just about any source. By early June 1972 she had succeeded in using her method to efficiently generate SV40-dvgal120 recombinant DNA. She was unable to clone these recombinants in E. coli, however, because of Berg’s moratorium on performing such an experiment. Nonetheless, Mertz’s discovery of an easy-to-use technique for constructing recombinant DNAs made it technically feasible for scientists throughout the world to begin joining together DNAs from different sources.

In June 1973, Herbert Boyer, of the University of California at San Francisco, reported that his laboratory, in collaboration with Stanley Cohen’s laboratory at Stanford University, had successfully used Mertz’s methods to generate and clone in E. coli a recombinant DNA encoding resistance to an antibiotic obtained from a different source of bacteria. This announcement led to a much more general moratorium covering many types of cloning experiments in numerous countries, including the UK and US However, the potential uses of these methods were clearly enormous. By the end of the 1970s numerous biotechnology companies had been formed and hundreds of researchers throughout the world had started cloning a wide variety of genes, including human ones for the manufacture of drugs such as insulin.

Meanwhile, Mertz continued to work on a variety of projects related to SV40, completing her doctorate in 1975. The remainder of her thesis focused on the development of other methods for the isolation and characterization of mutant variants of SV40. Berg’s laboratory subsequently employed these methods to show that SV40 could be used as a cloning vector to deliver and express genes in human cells. Paul Berg received the 1980 Nobel Prize in Chemistry in part for discoveries Mertz made under his mentorship.


Even before completing the writing up of her doctoral thesis, Mertz had been offered tenure-track faculty positions in four top-ranked US universities based on the reputation she had built up while working in Berg’s laboratory. They were also eager to have her join them because legislation passed in the US in 1972 (Title IX) put pressure on universities with government-funded grants to increase the number of women in their science faculties. In January 1975, Mertz chose to accept an offer from the McArdle Laboratory for Cancer Research at the University of Wisconsin-Madison, in part because it was a long-established, well-regarded centre for cancer research with excellent funding. In addition, Elizabeth Miller (a famous senior female scientist with numerous awards and honours) was a long-standing member of its faculty. Thus, Mertz would not be viewed as the token woman, simply hired because they needed to have one. The university also agreed to keep Mertz’s position open for two years so that she could complete her doctorate and do some post-doctoral research abroad before arrival.

In August 1975, Mertz took up a post-doctoral fellowship in the UK’s Medical Research Council Laboratory of Molecular Biology in Cambridge. Mertz was attracted to the place both because of its beautiful location and the fact that it was one of the very top-ranked molecular biology institutes in the world. She chose to work in the laboratory of John Gurdon so that she could learn from and complement his expertise in developmental biology. Alongside Gordon, Mertz also worked closely with Edward M DeRobertis, another postdoctoral fellow in his laboratory who specialised in protein biochemistry. Together, they developed among the first methods for successfully studying various aspects of gene regulation in a higher organism using oocytes (unfertilised eggs) obtained from the South African clawed frog. Prior to their work, most gene regulation studies had been conducted in bacteria and other single-celled organisms such as yeast. The oocyte system they developed is still used today for some studies. Mertz remained in Cambridge until December of 1976.

Once in Wisconsin, Mertz continued studying regulation of the expression of the genes of SV40. A key advantage in studying this virus was the fact that its genes are expressed at high levels in primate cells, including human ones. This provided a means to overcome some of the technical challenges associated with investigating gene expression in mammalian cells. An additional benefit of studying this virus was that it induces tumours in rodents and transforms mouse and human cells in culture to a cancerous state. This made it possible to study the roles viruses play in causing cancers. Mertz’s research subsequently extended to the hepatitis B and Epstein-Barr viruses, viruses associated with several types of cancers in humans. In recent years, Mertz has also focused some of her attention on the roles of oestrogen-related receptors in breast cancer.


Mertz played a major role in developing tools for the construction and cloning of recombinant DNAs which helped fuel the modern biotechnology revolution. One of her major contributions was discovering the DNA-cutting mechanism of EcoRI, a restriction enzyme originally purified from E. coli by Herbert Boyer's team. She was the first person to notice that whenever this enzyme cleaved the circular DNA genome of SV40, it left 'sticky ends'. Moreover, she discovered they could easily be brought back together by hydrogen bonding and resealed by treatment with another enzyme from E. coli, DNA ligase. This observation was a major breakthrough. In collaboration with Ronald Davis (an assistant professor in the Stanford University Biochemistry Department at that time), she further showed that a variety of DNAs contain the same sticky ends after cleavage with EcoRI. Based on this, she proposed that EcoRI together with DNA ligase could be used to easily join together just about any two pieces of DNA. She demonstrated this by using the method to make SV40-dvgal120 recombinants. This recombinant DNA was the one item she had lacked for her originally planned cloning experiment thwarted by the Berg moratorium following her attendance at the CSHL course in 1971. Her work with Davis was published in November 1972 in the Proceedings of the National Academy of Science, USA (vol 69, issue 11, pp. 2270-74).

Mertz’s laboratory notebooks show that she had all of the necessary methods and reagents needed to prove that recombinant DNA could be cloned in bacteria by early June of 1972. She was prevented from taking the very last actual cloning step, however, by Berg's suspension of experiments involving the cloning of SV40 DNA in E. coli. The US NIH Guidelines for Research Involving Recombinant Nucleic Acid Molecules eventually lifted this moratorium on cloning SV40 in 1979. In the interim, the first successful cloning of recombinant DNA in bacteria was achieved by Stanley Cohen and Herbert Boyer during the first half of 1973, a feat for which they were issued the first patents in this area. Their success was made possible in large part by Mertz’s earlier discoveries.

Mertz’s work not only laid important foundations for genetic engineering. The controversy that broke out over her first planned recombinant DNA experiment, together with the first successful cloning by Boyer and Cohen, set in motion events that led to the holding of the first international conference to discuss the bio-safety ramifications of genetic engineering. Held in Asilomar, California in 1975, this conference established the first safety guidelines for laboratories experimenting with recombinant DNA technology. Updated versions of these guidelines remain in place to this day.

In addition to her early work with recombinant DNA and frog oocytes, Mertz has been at the forefront of discoveries related to tumour viruses and oestrogen-related receptors. Within the context of human cancer viruses she has shown how they can regulate their switch from early-to-late or latent-to-lytic gene expression in part by utilising key cellular transcription factors, including hormone receptors such as the oestrogen-related receptors. Her work in this latter area is helping our understanding as to why some patients with some types of breast cancer respond poorly to hormone-based therapies such as tamoxifen. Her ongoing work with the Epstein-Barr virus may lead to the development of new combinations of drugs for treating patients with some types of lymphomas and carcinomas.

Lastly, Mertz, in a collaboration with her spouse Jonathan Kane, has challenged the commonly held belief that girls are not as good as boys in mathematics by analysing data from international mathematics competitions. She has found that female performance in mathematics at all levels correlates well with gender equity of the country in which they live, with the so-called gender gap due to changeable socio-cultural factors, not innate sex differences.

This profile was written by Lara Marks in September 2017 with generous input from Janet Mertz.


Interview with Janet Mertz by Stephanie Chen, 5 April 2013, Duke University Library.

Chen, S. 'Authorship and inventorship: An analysis of publishing and patenting norms and their consequences at American universities', Undergraduate PhD thesis, Duke Univesity, April 2014.

Friedberg, E. C. A biography of Paul Berg, London, 2014.

Kozubek, J. Modern Prometheus: Editing the Human Genome with Crispr-Cas9, Cambridge, 2016.

Yi, D. The Recombinant University, Chicago, 2015.

Janet Mertz: timeline of key events

1949-01-01T00:00:00+0000The aim of her docrtoal research was to figure out how to replicate and express recombinant DNA in E. coli. 1970-09-01T00:00:00+0000This was done in Dale Kaiser's laboratory by Douglas Berg together with Janet Mertz and David Jackson1971-01-01T00:00:00+0000Robert Pollack contacted Paul Berg to raise concerns about the potential biohazards of experiments Mertz, his doctoral research student, planned to do involving the introduction of genes from the oncovirus SV40 in the human gut bacteria, E. Coli. Following this Berg self-imposed a moratorium on experiments in his laboratory involving the cloning of SV40 in E-Coli.1971-06-01T00:00:00+0000It was based on their finding that when DNA is cleaved with EcoRI, a restriction enzyme, it has sticky ends. JE Mertz, RW Davis, 'Cleavage of DNA by RI restriction endonuclease generates cohesive ends', PNAS, 69, 3370–3374 (1972). 1972-11-01T00:00:00+0000Her thesis focused on methods to isolate and characterise mutant variants of SV40 1975-01-01T00:00:00+0000The method, known as the oocyte stem, was developed by Janet Mertz together with John Gurdon and Edward M DeRobertis. It was published in EM. De Robertis, JB. Gurdon, GA. Partington, JE Mertz, RA, 'Injected amphibian oocytes: a living test tube for the study of eukaryotic gene transcription?', Biochemistry Society Symposium, 42 (1977),181-91.1977-01-01T00:00:00+0000
Date Event People Places
1949Janet Mertz bornMertz 
September 1970Mertz started her doctorate in biochemistry at Stanford University under Paul BergMertz, Berg 
1971First plasmid bacterial cloning vector constructedBerg, Mertz, JacksonStanford University
June 1971Janet Mertz forced to halt experiment to clone recombinant DNA in bacteria after safety concerns raisedMertz, Berg, PollackStanford University
November 1972Janet Mertz and Ronald Davis published first easy-to-use technique for constructing recombinant DNA showed that when DNA is cleaved with EcoRI, a restriction enzyme, it has sticky endsMertz, DavisStanford University
January 1975Mertz completed her doctorate MertzStanford University
1977First method developed for studying gene regulation in a higher organismMertz, Gurdon, De RobertisLaboratory of Molecular Biology


Janet Mertz born

Sep 1970

Mertz started her doctorate in biochemistry at Stanford University under Paul Berg


First plasmid bacterial cloning vector constructed

Jun 1971

Janet Mertz forced to halt experiment to clone recombinant DNA in bacteria after safety concerns raised

Nov 1972

Janet Mertz and Ronald Davis published first easy-to-use technique for constructing recombinant DNA showed that when DNA is cleaved with EcoRI, a restriction enzyme, it has sticky ends

Jan 1975

Mertz completed her doctorate


First method developed for studying gene regulation in a higher organism

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