Some cancers have an abundance of mutations at cytosines on one DNA strand, which is consistent with the mutagenic properties of APOBEC cytidine deaminases. APOBEC3B mRNA expression has been found in various cancers, whereas the role of APOBEC3A as a mutator is less clear. By analysing 15 cohorts of recently published whole-genome sequencing samples that included 5 different types of cancer with mutation signatures resembling those of either APOBEC, Chan et al. have found that APOBEC3A is the predominant mutagenic deaminase. Although APOBEC3B mRNA tends to be more abundant than that of APOBEC3A, APOBEC3A is a much more potent inducer of mutations than APOBEC3B.