One of the most prominent mutation signatures in cancer, present in over half of human tumors, is called the APOBEC signature, or Signatures 2/13, and derives from the activity of the APOBEC3A (A3A) and APOBEC3B (A3B) cytosine deaminases, which preferentially deaminate cytosines in the context of an immediately preceding thymine (written TpC)1,2. This evidence concerns the gene APOBEC3A and cancer.