APOBEC proteins inhibit retroviruses by deaminating cytosine residues of viral RNA and DNA. The seven APOBEC3 genes or pseudogenes are found in a cluster thought to result from gene duplication on chromosome 22. APOBEC3 proteins are thought to be RNA editing enzymes and have roles in cell growth. APOBEC3B, also known as phorbolin-1-related protein or phorbolin-2/3, is a 382 amino acid protein belonging to the cytidine and deoxycytidylate deaminase family. APOBEC3B lacks cytidine deaminase activity on RNA molecules, but has been shown to bind to apoB and AU-rich RNAs. APOBEC3B forms a homodimer and interacts with APOBEC3G. APOBEC3B is expressed at at moderate levels in heart, testis, thymus, prostate, ovary, spleen and peripheral blood leukocytes.