The complete nucleotide sequence of mouse-truncated mRNA of c-kit, tr-kit, has been determined using the CD1 strain. In this study, the nucleotide sequences of tr-kit from AKR/N, C57BL/6, and ICR strains of mice were determined and found to be identical, although many silent variations were found compared with the sequence in a database for CD1. Tr-kit protein consists of 12 amino acids encoded by the 16th intron and the following 190 amino acids of c-kit. In the sequences of tr-kit encoding 12 specific amino acids, no substitution was detected among the three strains and CD1. Furthermore, RT-PCR analysis clearly showed that tr-kit mRNA expression was present only in testis. No nucleotide mutation in two important regions of the presumptive promoter for tr-kit mRNA was detected within the 16th intron of the mouse strains examined. However, no functional form of tr-kit was found in the rat, pig, or human by sequence analysis and homology testing.