DK/Lm is a new inbred mouse strain with over 20 generations of brother–sister mating. The genotype of the DK/Lm mouse at the black-brown locus is b/b and heterozygosity at the agouti locus (Ay/a) is maintained. DK/Lm-Ay/a mice become sable in phenotype at the first moult, whereas C57BL/6J-Ay/a mice do not. The sable phenotype is defined as that of a mouse whose basic colour is yellow (phaeomelanic) but whose dorsal aspect is more or less darkened by the presence of nonyellow (eumelanic) pigment. At about 6 months of age the DK/Lm mouse gradually reverts to yellow in phenotype.
Mice of the two strains are compared. Observations are discussed and related to hypotheses regarding gene action at the a and e loci. The new strain is a useful experimental model for study of the relationship between gene action at the agouti locus and the important pleiotropic effects influenced by this locus.