Required for activation of ATM following ionizing radiation. May act by regulating dephosphorylation of ATM. Defects in BRAT1 are the cause of rigidity and multifocal seizure syndrome, lethal neonatal (RMFSL). A lethal, neonatal, neurologic disorder characterized by episodic jerking that is apparent in utero, lack of psychomotor development, axial and limb rigidity, frequent multifocal seizures, and dysautonomia. At birth, affected individuals have small heads, overlapping cranial sutures, small or absent fontanels, and depressed frontal bones. Infants show poorly responsive focal jerks of the tongue, face and arms in a nearly continuous sequence throughout life. 3 isoforms of the human protein are produced by alternative splicing. Note: This description may include information from UniProtKB.