5-methyl cytidine has been indicated in the controlling mechanism for the genetic change, which leads to cancer. Human APOBEC3A (A3A), a polynucleotide cytidine deaminase (PCD) with specificity for single stranded DNA, can extensively deaminate human nuclear DNA. It was discovered that A3A among all human PCDs can deaminate 5-methylcytidine in a variety of single stranded DNA substrates both in vitro and in transfected cells almost as efficiently as cytidine itself. As 5MeCpG deamination hotspots characterize many genes associated with cancer it was made suggestion that A3A is a major player in the onset of cancer.