APOBEC3A (apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3A), also known as PHO1 or ARP3 or PHRBN is a 199 amino acid protein that belongs to the cytidine deaminase family. Expressed in keratinocytes, as well as in peripherical leukocytes, APOBEC3A may be implicated in cell growth and cell cycle control. The gene encoding APOBEC3A is one of seven related genes which map to a cluster on chromosome 22 and are thought to play a role in tumor transformation and metastasis, as well as in host-associated antiviral activity. Chromosome 22 houses over 500 genes, some of which are involved in Phelan-McDermid syndrome, schizophrenia and Neurofibromatosis type 2.