Most of the experimental studies have been carried out on murine melanoma cell lines, known to express between 3000 and 10 000 MSH binding sites per cell (Lambert and Lerner, 1983; Siegrist et al, 1989; Lunec et al, 1993), while human melanoma cells only express around 1000–3000 binding sites per cell (Ghanem et al, 1988). This evidence concerns the gene POMC and melanoma.