Rare mutations in LIG4 (encoding DNA ligase IV), XLF, DCLRE1C (encoding Artemis) or PRKDC (encoding DNA-PKcs) have been identified in a radiosensitive sub-class of patients with severe combined immunodeficiency (SCID) and can predispose to cancer (37, 38). The gene discussed is PRKDC; the disease is severe combined immunodeficiency.