TYR and melanoma: Assuming that proteolytic activity is required for cell migration in vivo; that tyrosinase synthesis reflects expression of the differentiated phenotype and that melanoma cells retain some of the characteristics of neural crest cells, we suggest that the effects of confluence and close cell-cell contact provide a useful experimental counterpart for the study of normal neural crest all behaviour that is characterized by an inverse relationship between migration and a protease secretion on the one hand and pigmentation on the other.