The mutation pale ears (ep) arose spontaneously in strain C3HeB/FeJ at the Jackson Laboratory in 1957. This mutation was maintained on this background by sibling inbreeding with forced heterozygosis by Dr. E. L. Green who subsequently backcrossed the mutation onto C57BL/6J, reaching generation N15 in 1968, and N20F18 in 1974. In 1972 Dr. Don Bailey reported finding numerous C57BL/6J congenic lines with marker genes not to be isocongenic with C57BL/6J by the usual test cross method. One of these histocompatibility loci was temporarily called H(ep) and was located about 16 units from ep (Bailey and Bunker, 1972). This temporary H(ep) locus was likely H51. Dr. Bailey received this strain at generation N20F3 and outcrossed once to C57BL/6By before again maintaining by sibling inbreeding. This strain was cryopreserved in 1979 by mating wildtype (+/+) females with homozygous males at generation B6JN20F3B6ByN1F19.