Although the genetic loss of Ku in mice is associated with genomic instability and increased incidence of lymphomas (Nussenzweig et al., 1996; Gu et al., 1997; Li et al., 1998, 2007), there has been presently no hereditary disease linked to the loss or inactivating mutations of the genes coding for Ku70 (XRCC6) or Ku86 (XRCC5) in humans, implying an essential function for Ku in human cell viability (Li et al., 2002; Fattah et al., 2008). The gene discussed is XRCC5; the disease is lymphoma.