To identify tumor-cell-intrinsic genes essential for CD8+ T cell killing we performed a genome-wide CRISPR/Cas9 deletion library screen in B16F10 melanoma cells expressing the model antigen SIY (SIYRYYGL) fused with DsRed (designated B16.SIY), under in vitro selection pressure by SIY-specific 2 C CD8+ TCR transgenic (Tg) cytotoxic T cells (Supplementary Fig. 1a). The gene discussed is CD8A; the disease is neoplasm.