Pigmentation is a complex process that provides many diverse benefits for the organism. Pigment production takes place in the pigment cell lineage in dedicated organelles called melanosomes. Several proteins are specifically recruited to melanosomes for this purpose, and many of them are involved in melanin synthesis. MART-1 is one of the melanosomal proteins. It has been widely studied as an agent for immunotherapy against malignant melanoma but very little is known about the gene and its function. Here, I used different approaches in order to understand the biology of this pigment cell specific gene. First, I analyzed transcriptional regulation of the mouse MART-1 gene. I found that the shortest promoter region (0.6kb) alone is not sufficient to drive the expected MART-1 expression in vivo, even though it was able to express a luciferase reporter in vitro, in transfected cells. The largest promoter region (6.0kb) was able to produce expression of a LacZ reporter gene in the RPE layer starting from the early stages of embryogenesis. However it failed to drive melanoblast-specific expression in embryos and, only after birth, reporter gene expression in melanocytes became visible, indicating that distal regulatory elements might be involved in melanocyte/melanoblastspecific MART-1 expression. In order to visualize the spatial and temporal expression pattern of MART-1, I developed a MART-1::LacZ BAC reporter mouse and showed that in mouse, MART-1 expression starts at around E11.5-12.5, and its expression is exclusive to melanocytes and RPE. To understand the function of MART-1, I developed a knockout mouse model. I have shown for the first time that MART-1 is directly involved in pigmentation and that its loss leads to coat color dilution. It has been reported that MART-1 might be involved in trafficking and processing of the melanosomal proteins Pmel17 and OA1. I analyzed the distribution of Pmel17 and OA1, as well as other melanosomal proteins in melanocyte lines derived from MART-1 knockout mice, however no mislocalizations were observed. Besides, in primary knockout melanocytes, the processing of Pmel17 was not affected. Nevertheless, electron microscopy studies revealed that MART-1 knockout mice had severe morphological defects in melanosomes of the hair follicle melanocytes. This indicated that the coat color dilution observed in MART-1 knockout mice might be due to the structural defects in the melanosomes, suggesting that MART-1 is required for melanosome biogenesis in the hair follicle.