Checkpoint Kinases (Chk) are protein kinases that are involved in cell cycle control. Two checkpoint kinase subtypes have been identified, Chk1 and Chk2. Chk1 is a central component of genome surveillance pathways and is a key regulator of the cell cycle and cell survival. Chk1 is required for the initiation of DNA damage checkpoints and has recently been shown to play a role in the normal (unperturbed) cell cycle. Chk1 impacts various stages of the cell cycle including the S phase, G2/M transition and M phase. In addition to mediating cell cycle checkpoints, Chk1 also contributes to DNA repair processes, gene transcription, embryo development, cellular responses to HIV infection and somatic cell viability. Chk2 is a protein kinase that is activated in response to DNA damage and is involved in cell cycle arrest. In response to DNA damage and replication blocks, cell cycle progression is halted through the control of cell cycle regulators. The protein encoded by this gene is a cell cycle checkpoint regulator and putative tumor suppressor.