Chk (Checkpoint kinases) are protein kinases that are involved in cell cycle control. Two checkpoint kinase subtypes have been identified, Chk1 and Chk2. Chk1 is an Serine/threonine-specific protein kinase that in humans, is encoded by the CHEK1 gene. Chk1 coordinates the DNA damage response (DDR) and cell cycle checkpoint response. Activation of Chk1 results in the initiation of cell cycle checkpoints, cell cycle arrest, DNA repair and cell death to prevent damaged cells from progressing through the cell cycle. Chk1 impacts various stages of the cell cycle including the S phase, G2/M transition and M phase.CHEK2 is the official symbol for the human gene Checkpoint kinase 2. CHEK2 is located on the long (q) arm of chromosome 22. CHEK2 is tumor suppressor gene that encodes the protein CHK2, a serine threonine kinase. CHK2 operates in an intricate network of proteins to elicit DNA repair, cell cycle arrest or apoptosis in response to DNA damage. Mutations to the CHEK2 gene have been linked to a wide range of cancers including breast cancer.