Tumour suppressor genes (e.g. TP53, RB1) normally restrain cell proliferation and promote apoptosis. Tumour promoters can activate oncogenes indirectly by inhibiting the expression or function of suppressor genes, removing the brakes on uncontrolled cell division. This is a key mechanism in the two-stage model of carcinogenesis (initiation → promotion). Promoters are not directly mutagenic but enhance the effect of initiators.