The classical discrete, stepwise model of tumour suppression (left) is contrasted with a continuum model of tumour suppression and oncogenesis (centre and right, respectively). In the discrete model, tumourigenesis is induced by either complete loss of a TSG (two-hit paradigm, dark blue) or after single-copy loss of a TSG (haploinsufficiency, light blue). In contrast, we propose a continuum model (centre and right), in which tumour suppression is related to a continuum of TSG expression, rather than to discrete changes in DNA copy number. A continuum of increasing TSG expression will generally be negatively correlated with malignancy (centre, light green) whereas increasing oncogene expression will generally be positively correlated with malignancy (right, red). A linear relationship is depicted for schematic purposes, but the dose-response relationship need not be linear. In some cases, fail-safe mechanisms are induced by complete loss of TSG expression or by massive oncogene overexpression. In these cases, complete loss of TSG expression (centre, dark green) or massive overexpression of an oncogene (right, orange) will be negatively correlated with malignancy, as shown.