Activation-induced cytidine deaminase (AID) plays an essential role in the generation of a highly competent repertoire<br /> of antibodies by participating in class switch recombination (CSR) and somatic hypermutation (SHM). After<br /> B cell stimulation by antigens, AID initiates SHM and CSR by deamination of cytidine to uridine in the variable<br /> and constant regions of Ig genes. An adverse effect of AID’s ability to directly modify genome sequences is its<br /> mutagenic potential. AID has been shown to occasionally target non-Ig genes and its abnormal expression is<br /> strongly associated with tumorigenesis. Aberrant AID expression can also be triggered by some oncogenic pathogens,<br /> such as <i>Helicobacter pylori</i> and several viruses. The latest reports show that also under physiological conditions<br /> AID might act beyond the immune system. Data exist suggesting that AID can play a role in the process<br /> of active genome demethylation – the heart of epigenetic gene activation and reprogramming. Moreover, it has<br /> been shown that abnormal genome demethylation mediated by AID might be associated with human colon cancers.<br /> The potential role of AID in the active demethylation process is still controversial, but the hypothesis that<br /> aberrant AID expression may cause cancerogenesis by changing genome methylation patterns appears highly<br /> attractive. As a unique human enzyme able to induce both genetic and epigenetic alterations under physiological<br /> and pathological conditions, AID could be a promising and versatile drug target. In this review we present the<br /> current state of knowledge on this topic and the controversies surrounding the pleiotropic effect of AID function.