The Black 6 or the C57BL/6 mouse is often used in life extension studies and the lifespan of this strain is well documented. This type of mouse also has the distinction of being the second mammalian species after Humans to have its entire DNA sequenced back in 2002. The black 6 mouse, was developed around 1920 by Clarence Cook Little (1881-1971). As a boy, Clarence Cook Little kept mice as pets, then later he began studying the inheritance of mouse coat color at Harvard University. Around this time Wilhelm Johannsen, the Danish botanist who coined the word "gene," had shown the value of inbreeding to set characteristics. As a result Little applied this idea to mice and in 1909, he produced the first inbred mouse strain, known as DBA, by repeatedly mating brother-sister pairs. Little sought to sort a trait far more complex than coat color, this was cancer susceptibility. The DBA strain, however, showed certain frailties from inbreeding that compromised its usefulness especially when more robust mouse strains arrived.
Around 1914, Little obtained a female mouse, code numbered 57, and a male, code numbered 52, which he inbred through at least twenty generations. He named the resulting strain C57BL/6. The number 6 refers to one of multiple lines within the strain, among others that did not survive inbreeding pressures. Although C57BL/6 or "black six" had some inherited weaknesses, it was easy to breed, and also showed delayed senescence of its hematopoietic system compared to DBA.
Today this mouse type has become so popular that if there was a standard mouse that researchers use the Black 6 would be that mouse, a wealth of tests and studies using this strain give us a great deal of data to use. It is the top selling mouse strain at Charles River and many other breeders due to its relative robustness, ease of breeding and is easy to care for. The Jackson mouse breeding lab note that this is the most widely used mouse strain they sell here along with other interesting facts about this particular mouse.
Fun fact: Black 6 is one of the few mouse strains that will willingly drink alcohol and is in fact quite fond of a drink if given the opportunity!