Such investigations have revealed that the inactivation of mismatch repair genes, inactivating mutations of BRCA1 (ENSG00000012048), BRCA2 (ENSG00000139618), POLE (ENSG00000177084), and POLK (ENSG00000122008), or the overexpression of APOBEC3B (ENSG00000179750) causes mutagenesis in cancer [13–17]. The gene discussed is APOBEC3B; the disease is cancer.