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let’s say hypothetically, I wanted some mice with specific knockout that wasn’t available – would I need to do that myself or could it be contracted to another lab?’
Today this can be easily contracted to a commercial lab such as Jackson Laboratory (JAX) or Taconic Biosciences (Genetically Engineered Rodent Models | Precision Research Models).

Delving briefly into the history of mice in research will help understand why and how so many knockout and transgenic mice came into existence over the past 25 years or so.

  • Biomedical research enterprise as it exists today wasn’t yet in existence a century back when individual mouse fanciers like Abbie Lathrop set about establishing vast mouse colonies in their homes as a business to cater to hobbyists and those seeking exotic pets. What’s interesting about a person like Abbie is that though untutored in academic science, she kept careful breeding records and established early inbred mouse strains, all this pre-WW I (1).
  • Availability of such inbred mouse strains in turn caught the attention of pioneering scientists working on mammalian genetics, who made it the foundational piece of their research efforts, so much so that post-WW II, slowly the mouse took over and displaced rat and guinea pig as the animal model of choice.
  • Apart from established inbred strains, another reason for mouse taking over post-WW II was also simple economics. Easier to feed, breed and house, mouse, with its shorter generation times and relatively larger litters, had key economic advantages over rat and guinea pig.
  • Mammalian genetics pioneers who helped establish the mouse at the heart of the biomedical research enterprise included William E. Castle, George Davis Snell, and perhaps most importantly for the mouse enterprise, C. C. Little.
  • CC Little or CC for short set up the JAX mouse breeding facility in Bar Harbor, Maine, USA. JAX is today one of the most important sources of inbred mouse strains worldwide.

So that’s a very brief history on how and why inbred mouse strains became the bread and butter of basic biomedical research.

Next, transgenic and knockout mice.

  • Mouse strains entered their next, even more exponential phase of expansion in the 1980s when scientists such as Mario Capecchi, Martin Evans, Oliver Smithies, among others, developed the techniques of in vitro Gene targeting using Homologous recombination. This technique enabled the rapid creation of transgenic and knockout mice, by inserting a gene into the mouse genome, i.e., Transgenesis, or by knocking out specific genes, i.e., Knockout mouse.
  • When it comes to transgenics and knockouts, a particular obscure mouse strain became extremely important. This was the 129 strain. Why? Because some of its sub-strains, in particular 129/Sv, were spectacular in their ease of ability to yield embryonic stem (ES) cell cultures.
    • Ease of establishing such cultures and manipulating them in vitro proved a bonanza for genetic engineering (2, 3, 4, 5, 6).
    • On the one hand, such tremendous ease in inserting and knocking out genes could greatly accelerate advances in studies of diseases and syndromes.
    • OTOH, the 129 mouse strain was not a good candidate to enable such studies.
    • By the 1980s, C57BL/6 (B6) and BALB/c had become the staples in mouse studies but not 129. Why? The former were much more fecund and robust. For e.g., litter sizes of 6 to 10 are easily possible with B6 and BALB/c compared to barely 1 to 3 with 129.
    • So empirically the process evolved to make transgenics and knockouts on the 129 background, i.e., using 129 ES cells, implanting them in pseudopregnant mice of a chosen inbred mouse strain and then painstakingly back-crossing the progeny to the inbred mouse strain of choice over several generations. Typically, backcrosses of 10 to 12 generations are necessary to ensure that the transgenic or knockout has >99% of the selected inbred mouse strain’s genes.
  • At this point, say in late 1980s and early 1990s, the labs that were the first to establish an assembly-line like approach to mouse transgenesis and knockout technology understandably reaped the windfall benefits.
    • For e.g., in immunology there was a period around the 1980s and 1990s when any high profile paper worth its salt would list as one of its authors a researcher like Tak Wah Mak. Why? Simple because he was Mr. Knockout.
    • This state of affairs continued until well into the 2000s with researchers such as James P. Di Santo who created extremely important knockouts such as the common gamma chain knockout reaping the benefits in terms of an avalanche of papers.
    • So there was a period of about 15 to 20 years when researchers depended on the good will of colleagues in academic centers to provide them with a breeding pair or two of specific transgenic and knockout mouse strains.
    • The system of making transgenics and knockouts improved with the development of Conditional gene knockout using the Cre-Lox recombination process.
    • Over time, researchers deposited founding pairs of mouse transgenics and knockouts they created with large animal model suppliers.

Thus, today, transgenic and knockout mice have become routine and are literally a mouse click away on the computer. All one has to do is browse the catalog on the web-site of big time suppliers such as JAX, Taconic and Charles River Laboratories (Charles River Laboratories | Every Step of the Way.), which are three of the largest such suppliers in the US.

Next big step in this process will likely be big expansion of the CRISPR/Cas9 (Gene drive) system into mouse transgenesis and knockouts. I predict this will greatly accelerate the rate of transgenic and knockout mouse strains since researchers would no longer have to rely on the convoluted process of creating them in 129 ES cells initially and then painstakingly back-crossing them into the inbred mouse strain of their choice.


  1. Steensma, David P., Robert A. Kyle, and Marc A. Shampo. “Abbie Lathrop, the “mouse woman of Granby”: rodent fancier and accidental genetics pioneer.” Mayo Clinic Proceedings. Vol. 85. No. 11. Mayo Foundation, 2010. Page on nih.gov
  2. Stevens, L. C., and K. P. Hummel. “A description of spontaneous congenital testicular teratomas in strain 129 mice.” Journal of the National Cancer Institute 18.5 (1957): 719-747.
  3. Martin, Gail R. “Isolation of a pluripotent cell line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells.” Proceedings of the National Academy of Sciences 78.12 (1981): 7634-7638. Page on pnas.org
  4. Baribault, Helene, and R. Kemler. “Embryonic stem cell culture and gene targeting in transgenic mice.” Molecular biology & medicine 6.6 (1989): 481-492.
  5. Bradley, Allan, et al. “Genetic manipulation of the mouse via gene targeting in embryonic stem cells.” Postimplantation Development in the Mouse. Wiley West Sussex, 1992. 256-276.
  6. Bradley, A. L. L. A. N., B. I. N. H. A. I. Zheng, and P. E. N. T. A. O. Liu. “Thirteen years of manipulating the mouse genome: a personal history.” International Journal of Developmental Biology 42 (1998): 943-950. Page on ijdb.ehu.es