The grey-lethal (gl) mouse is the most relevant animal model for recessive osteopetrosis, a genetic defect affecting bone resorption. To localize the gl gene, two novel backcrosses between the gl mutant strain GL/Le dlJ +/+ gl and with the Mus spretus or the Mus m. molossinus have been generated and typed with 19 DNA markers representative of genes or microsatellites. In the Mus m. molossinus backcross, the gl locus cosegregates with the D10Mit108,109,184,193, 254,255 markers within a 1 centimorgan genetic interval between the markers (D10Mit54,55,215,Cd24a) and D10Mit148. Our results have also eliminated all the five candidate genes previously localized to this region (Braf-rs1, Fyn, Cd24a, Ros1, and Gja1). On the Mus spretus background, segregation distortion due to a approximately threefold differential survival resulted in a severe deficit in gl/gl animals, indicating the presence of modifier genes. We have also characterized nine cosegregating microsatellite markers closely linked to gl as defined by their specific polymorphisms for the Chromosome (Chr) 10 harboring the gl mutation. Screening of several mouse inbred strains for these polymorphic markers revealed an identical pattern between gl and 129/SvEms, suggesting that the gl mutation arose on this genetic background. The linkage between this polymorphic region and the gl locus provides an entry point to produce a physical map to isolate the gl gene.