In one of these studies it was demonstrated that nude mice bearing either mouse 66 (mammary carcinoma) or MDA-MB-435 cells (human melanoma) injected with the fluorescently-labeled sigma-2 ligand SW120 was taken up selectively by proliferating tumor cells and not by the generally quiescent peripheral blood mononuclear cells [21]. This evidence concerns the gene AP2S1 and melanoma.