Bill Russell (1951) created a landmark in the field of mouse genetics by creating a specifically designed mouse strain, the "T" (test) stock that was used in genetic screens for testing mutagens such as radiations and chemicals. The "T"-stock mouse harbors 7 recessive, viable mutations affecting easily recognizable traits. At the Oak Ridge National Laboratory, Russell's initial goal was to determine the rate of inheritable gene mutations in the germ line induced by radiations. Thus he decided to use "T"-stock mice in order to define how often a set of loci could be mutated with radiations. Since the mutations in the "T"-stock mouse were recessive, the progeny would have a wild type phenotype (as a result of crossing a mutant mutant male to a wild type female ["+"/"+"]). Thus with any progeny carrying a mutation induced by radiation at one of the 7 loci, would exhibit the mutant phenotype in the first generation itself. This approach, the specific locus test (SLT) allowed Russell to study a wide range of specific mutations and to calculate the mutation rates induced by radiations.