Ataxia angiectasia （AT） is a debilitating neurodegenerative disease that occurs early in childhood, resulting in ataxic movements and speech defects caused by cerebellar degeneration. The underlying cause of the disease is a biochemical dysfunction in the cellular response to specific types of DNA damage, correlated with mutations in the protein kinase ATM （Ataxia angiectasia mutated）. ATM not only is key in the signaling cascade that responds to DNA double-stranded breaks, but it also controls cell-cycle checkpoints and is therefore important in cancer prevention. It belongs to the PI3/PI4-kinase family. This protein is an important cell cycle checkpoint kinase that phosphorylates; thus, it functions as a regulator of a wide variety of downstream proteins, including tumor suppressor proteins p53 and BRCA1, checkpoint kinase CHK2, checkpoint proteins RAD17 and RAD9, and DNA repair protein NBS1. This protein and the closely related kinase ATR are thought to be master controllers of cell cycle checkpoint signaling pathways that are required for cell response to DNA damage and for genome stability. Mutations in this gene are associated with ataxia angiectasia, an autosomal recessive disorder. Two transcript variants encoding different isoforms have been found for this gene.