Most of the cells in an adult organism possess the same DNA content and the same complement of genes, yet we can recognise many different cell types in the adult. Development is the process by which the cells of the embryo progressively acquire their adult fate and position. The genetic network which controls these processes is beginning to be unravelled at an increasing pace, yet one key area is still somewhat neglected--namely, developmental cell memory, the process by which cells record their ontogeny. A significant component of the phenotype of cancer cells may be explained by deregulation of genes whose normal role is to control the division, differentiation and migration of embryonic cells during development. It should therefore not be surprising that genes implicated in cell memory processes during development are also implicated in disease. In this review we outline what is known about the Polycomb and trithorax group of proteins as candidate genes for the memory process, both in terms of basic functions and the roles of abnormalities in these genes resulting in cancer.