in addition to the retina, trpm1 is also expressed in melanocytes, which are melanin-producing cells in the skin. the expression of trpm1 is inversely correlated with melanoma aggressiveness, suggesting that it might suppress melanoma metastasis. however, subsequent work showed that a microrna located in an intron of the "trpm1" gene, rather than the trpm1 protein itself, is responsible for the tumor suppressor function. the expression of both trpm1 and the microrna are regulated by the microphthalmia-associated transcription factor.