Genetically, congenital syndromes that predispose to cancer, are always defective in certain genes encoding DNA repair proteins; biochemically, cancer cells are always defective in certain DNA repair pathways; and biologically, cancer cells express the mutator phenotype, including elevated single nucleotide alterations and chromosome structure aberrations (i.e., genomic instability), which suggests that the DNA has been mis-repaired, indicative of DNA repair defects. The gene discussed is XRCC6P5; the disease is cancer.