The major gene that causes cleft lip in the A/WySn mouse strain is being transferred to the unrelated normal AEJ/GnRk strain background by the backcross-testcross method. The result, a congenic strain pair differing only at the major cleft lip locus and closely linked loci, will allow the unequivocal identification of the genetic lesion that causes cleft lip. Four backcross generations have been completed, and the background geneotype is calculated to be 97% of AEJ/GnRk origin. The recovery of the cleft lip trait in testcrosses of each generation is compatible with the expected values for the transfer of a single major gene with 8% penetrance. Multilocus inheritance beyond two loci is ruled out. In separate experiments using mostly Robertsonian chromosomes as markers of the normal allele at the cleft lip locus, portions of the genome have been screened for the map location of the cleft lip gene. The gene is probably not within 10-20 cM of the centromere of 12 different identified autosomal chromosomes nor linked to Ph on chromosome 5. Previous studies have suggested that the cleft lip gene is not linked to c (chromosome 7), b (chromosome 4), d (chromosome 9), T, H-2 (chromosome 17), or N (chromosome 15).