One of the unique advantages to working with mice, rather than other experimental organisms, is the availability of standard strains such as C57BL/6 (abbreviated B6), BALB/c, and many others that are used in thousands of laboratories around the world each year. With the use of the same standard inbred strain, it is possible to eliminate genetic variability as a complicating factor in comparing results obtained from experiments performed in Japan, Canada, Germany, or any other country in the world. Furthermore, for the most part, results obtained in 1992 can be directly compared to results obtained in 1962 or any other year. But, where do these standard strains come from and how can one be sure that a mouse advertised as BALB/c is actually a BALB/c mouse?
When two animals have the same strain name, such as BALB/c, it means that they can both trace their lineage back through a series of brother-sister matings to the very same mating pair of inbred animals. The breeding protocol through which these original progenitors became inbred is discussed later in this chapter. However, the important point is that unlike the world of computers, where there can be many independent imitation models of a standard such as the IBM PC, there is no such thing as an imitation BALB/c mouse. Two animals either have a common heritage or they do not. If not, they cannot share the same name. Thus, a strain name implies a history, and the histories of the traditional inbred strains are well-documented (see Table 3.1).
A handful of US suppliers provide various strains of mice to researchers. Addresses and phone numbers for each are provided in Appendix A; all will provide free catalogs upon request. The Jackson Laboratory (or the JAX as it is commonly abbreviated) maintains an extensive mouse breeding facility with a very large collection of commonly used (and not so commonly used) strains for sale to other scientists. 12 Their 1991 catalog lists hundreds of different inbred strains and substrains of many different types including all of the "standards" as well as newly developed strains and mice that carry various mutant alleles or chromosomal aberrations. Other U.S. suppliers have a more limited selection, but the largest of these — Charles River Laboratory, Taconic Farms, and Harlan Sprague Dawley — may actually sell even more mice than the JAX. Each of these three companies stocks a set of common inbred strains — including BALB/c, C57BL/6, C3H, DBA/2 and several others — as well as hybrids and non-inbred strains. Two other U.S. companies — Hilltop Lab Animals and Life Sciences, Inc. — have more focused lists of strains with the latter devoted essentially to the sale of special athymic strains used in various immunological studies. All of these suppliers provide high quality, disease-free animals that are constantly monitored for genetic purity.
Once a supplier has been chosen for a particular set of experiments, it is best to stay with that supplier for all future orders of mice. The reason for this is that even though all suppliers propagate their stocks with constant brother-sister matings, and B6 mice sold by JAX, Charles River, or Taconic can all trace their pedigree back to common founder animals, it is still the case that each independently-maintained line will slowly drift apart genetically from its ancestors and distant cousins. Most of the standard inbred strains sold by companies are derived from a genetic resource maintained by the National Institutes of Health (NIH). The NIH inbred lines have been maintained separately from corresponding Jackson Laboratory lines since at least the early 1950s. The B6 strain was only at generation F32 when this separation occurred; by the beginning of 1994, the JAX strain had reached generation F187 and NIH-derived strains sold by Taconic and Charles River had reached generations 155 and 160 respectively (Table 3.1). The differences that have accumulated over this large number of generations may, or may not, have an impact upon the particular genetic characteristics of importance to any particular experiment, but it is critical to be aware of this possibility. To foster this awareness, independently-maintained inbred strains are given different "substrain" designations which follow the standard name and provide an account of past history. For example, the full name for the standard B6 mouse sold by the Jackson Laboratory is C57BL/6J where J is the symbol for JAX. The B6 mice sold by both Charles River and Taconic have substrain symbols with multiple parts including N as an indication of their NIH derivation and BR to indicate that they are maintained in barrier facilities. Each supplier has also incorporated a sub-symbol that uniquely identifies the animals that they sell — the full names are C57BL/6NCrlBR for B6 mice supplied by Charles River Laboratory and C57BL/6NTacfBR for B6 supplied by Taconic Farms.