However, the expression of MHC molecules on CTVT cells is important, with CTVT cells switching from an MHC-negative phenotype during tumour growth to MHC-positive during tumour regression (see below for further discussion).56,60,61 Cytokines are also thought to play a role.62 Higher concentrations of the pro-inflammatory cytokines IL-6 and IFN-γ are detected in ex vivo cultures of tumour-infiltrating lymphocytes from regressing tumours compared with growing tumours and the presence of these cytokines increases cytotoxicity of NK cells to CTVT cells in vitro62 (Fig.2). This evidence concerns the gene IFNG and neoplasm.