Checkpoint signalling pathways arrest the cell cycle when genomic integrity is threatened, preventing the transmission of genetic mutations into subsequent cell generations.The restriction point also regulates cell cycle progression based on environmental signals (growth factors, extracellular matrix attachment, cell–cell contacts, etc.). Because of these critical functions, mutations in key checkpoint genes contribute to a variety of human diseases. Notably, most malignant cancers possess mutations in one or more checkpoint genes and are genetically uns (Lengauer et al., 1998).