Contains 2 HEAT repeats. Ubiquitously expressed. Required for activation of ATM following ionizing radiation. May act by regulating dephosphorylation of ATM.The HEAT repeat is a tandemly repeated, 37-47 amino acid long module occurring in a number of cytoplasmic proteins, including the four name-giving proteins huntingtin, elongation factor 3 (EF3), the 65 Kd alpha regulatory subunit of protein phosphatase 2A (PP2A) and the yeast PI3-kinase TOR1. Arrays of HEAT repeats consists of 3 to 36 units forming a rod-like helical structure and appear to function as protein-protein interaction surfaces. It has been noted that many HEAT repeat-containing proteins are involved in intracellular transport processes. In the crystal structure of PP2A PR65/A, the HEAT repeats consist of pairs of antiparallel alpha helices.