Masts cells are progeny of the hematopoietic stem cell. For the differentiation of mast cells, a transcription factor encoded by the mouse mi locus (MITF) plays an important role. The expression of many genes encoding proteins that are essential for the function of mast cells is regulated by MITF. Because various mutant mice are available at the mi locus and because cultured mast cells are easily obtained from the spleen of these mutant mice, this system may be a good model for studying the regulation of hematopoietic cell differentiation by a transcription factor.