NODAL and melanoma: Several studies have reported that Nodal expression positively correlates with melanoma tumor progression toward a metastatic phenotype; moreover, its over-expression is paralleled by that of the co-receptor Cripto-1, suggesting the occurrence of tumor growth mechanisms supported by the concomitant presence of both proteins and by their mutual interaction with the activin-like receptor complex [5,9,10,11,12,13,22,23].