Since our aim was to isolate and identify new progression markers of human cutaneous melanoma, we applied the differential hybridization technique, in which we compared the gene expression in two subsequent stages of this progression. Tumors in nude mice arising after transplantation and serial passage in vivo of either the horizontally and early vertically growing part or the advanced vertically growing part of a primary melanoma of the same patient were used for this assay. This resulted in the isolation of a number of complementary DNA clones that were differentially expressed. Based on the marked difference in expression, one of them, designated pMW1, was chosen for further characterization and appeared to be coding for calcyclin, a cell cycle-regulated protein, belonging to a family of small calcium-binding proteins. Calcyclin expression was elevated in high-metastatic human melanoma cell lines in nude mice compared to low-metastatic ones. Immunoprecipitation of calcyclin showed that the differential expression at the RNA level is also reflected at the protein level. These findings show that expression of calcyclin is related to metastasis of human melanoma cell lines in nude mice and emphasize the role of this family of calcium-binding proteins in neoplastic progression as was reported for the mouse homologue of calcyclin and other members of the same family.