The mi transcription factor (MITF) is a basic helix-loop-helix leucine zipper (bHLH-Zip) transcription factor and encoded by the mi locus of mice. Double gene dose of mutant allele at the mi locus results the decrease of mast cells and phenotypic abnormalities of mast cells. Various mutations have been reported at the mi locus. We divided them to null and inhibitory mutations. The tg is a typical null mutation due to the insertion of a transgene into the promoter region of MITF gene. Adult mice of tg/tg genotype can easily obtained and are a potentially useful tool for studying development and function of mast cells.