XRCC4 and cancer: In our patients, undetectable levels of XRCC4 caused neither immunologic defects nor increased susceptibility to cancer, and were instead associated with a relatively mild syndrome, including moderately severe congenital malformations, and a slowly progressive, adult-onset combination of cardiac failure, due to dilating cardiomyopathy, and neurological impairment characterized by cerebellar failure, cognitive arrest/decline, and behavioral abnormalities.