Rb(4.15)1Rk was found in a male homozygous for In(1)12Rk. In(1)12Rk was identified in the offspring of a C57BL/6J female bred to a DBA/2J male that was treated with 700-900 R whole body x-irradiation. How this line was maintained prior to the identification of Rb(4.15)1Rk is not known. Rb(4.15)1Rk was backcrossed onto C57BL/6J for 10 generations then intercrossed to generate a homozygous line, which was maintained by sibling inbreeding. In 1993 embryos were generated for cryopreservation from C57BL/6J males bred to homozygous females at generation N10F21.