Mi protein encoded at the mouse microphthalmia (mi) locus is a transcription factor with a basic helix-loop-helix/leucine zipper structure. To assess the function of the human homolog of Mi protein, termed microphthalmia-associated transcription factor (MITF), we analyzed the effects of MITF on the promoter function of the mouse tyrosinase and tyrosinase-related protein 1 (TRP-1) genes. These two gene promoters are able to direct transcription preferentially in melanin-producing cells, and an enhancer element M box of 11 bp, containing a CATGTG motif, is conserved in both promoters. By transient expression assays, we have localized the cis-acting element of the tyrosinase gene responsible for pigment cell-specific expression to the proximal 82-bp region, which contains a CATGTG motif (positions -12 to -7) but lacks the M box (positions -107 to -97). We also provide evidence that the 82-bp region and the M box are involved in the transactivation of the tyrosinase promoter by MITF and that the M box is bound by MITF in vitro. Furthermore, MITF activated the TRP-1 gene promoter possibly through the M box (positions -44 to -34). These results suggest that MITF is a common factor regulating transcription of the pigment cell-specific genes.