The direct sequencing of the Kit cDNA obtained from mutant mice was used to reveal the molecular nature of the W−3Bao ENU-induced mutation. There was a T to A transversion at the 441st nucleotide in the W−3Bao open reading frame (ORF), which introduced a pre-mature termination codon at residue 147. The gross embryonic development, hematopoiesis and spermatogenesis were examined in the mutant mice. There was no visible difference among the W−3Bao/+, W−3Bao/3Bao and wild type embryos before embryonic day 12.5. W−3Bao/3Bao embryos appeared pale after E14.5 and dwarf after E16.5. An extremely low level of hematochrome and large red blood cells were found in W−3Bao/3Bao 18.5 days old embryos, leading to the stillbirth of the homozygotes. In 18.5 days old embryos the spermatogonia of W−3Bao/3Bao embryos did not migrate to the contorted seminiferous tubules properly, but instead were found in the interstitial tissue. The spermatogonia of W−3Bao/+ or W+/+ mice were present in both the interstitial tissue and contorted seminiferous tubules. In the adult male hetereozygotes, there are contorted seminiferous tubules with no spermatogonia, suggesting that the migration defect was dominant. In female W−3Bao/3Bao ovaries, primordial follicles were absent while primordial follicles appeared clearly in the ovaries of W−3Bao/+ or W+/+ mice. With a nonsense mutation in the Kit gene, W−3Bao/+ mice show white spotting and an abnormal development of the contorted seminiferous tubules and W−3Bao/3Bao mice are stillborn due to severe macrocytic anemia, and have abnormal genital glands in both the male and female.