Intriguingly, although ZD55-IL-24 could not kill B16 cells in vitro, it could robustly inhibit proliferation (Fig. 1I, J, M–O) and greatly promote apoptosis (Fig. 1K, L) in B16 cells within tumor tissues, suggesting that ZD55-IL-24 inhibited melanoma growth in B16-bearing immunocompetent mouse model not through the well-known direct pathway of inducing tumor-selective cytotoxicity, but through an unknown indirect pathway. This evidence concerns the gene IL24 and melanoma.