The gene MTS1 encodes p16INK4, an inhibitor of cyclin-dependent kinase 4, and is frequently deleted, mutated, or silenced by promoter methylation in melanoma cells and in the germline of familial melanoma patients. Although MTS1 may thus be the candidate melanoma suppressor gene that maps to chromosome 9p21, it is not clear how dysfunction at that locus temporally relates to melanoma progression. To further test its role in sporadic melanoma, the expression of p16INK4-protein and -mRNA was characterized in melanomas and melanocytic nevi by immunocytochemistry and in situ reverse transcriptase-polymerase chain reaction. Histologic tissue sections were immunolabeled with anti-p16INK4 antibody for 108 melanocytic lesions, including common and atypical nevi, in situ melanomas, primary invasive melanomas, and metastatic tumors. A subset of the lesions was analyzed for expression of p16INK4-mRNA, employing forward and reverse intron-bridging primers for reverse transcriptase-polymerase chain reaction amplification of the transcript corresponding to exons 1 and 2 of MTS1. Strong immunolabeling was detected in the melanocytes of common nevi and of nevi with architectural disorder and cytologic atypia. By digital image analysis, in contrast, labeling intensity decreased significantly and progressively in the melanocytes of in situ, invasive, and metastatic melanomas. Results from the in situ reverse transcriptase-polymerase chain reaction analysis were confirmatory, showing a strong signal in the melanocytic nevi but progressive signal attenuation with increasing stage of melanoma. These data indicate correlation between gradual loss of expression of the MTS1 locus and progression of melanoma, further supporting an emerging role for the gene in the malignant transformation of melanocytes. The failure to demonstrate reduced expression in nevi suggests either that these lesions are not an early stage in melanoma development, in contrast to prevailing assumptions, or that loss of p16INK4 function is not an initiating event in melanocyte transformation.