Somatic hypermutation (SHM) of the Ig genes is required for affinity maturation of the humoral response to foreign antigens. Activation-induced cytidine deaminase (AID), which is specifically expressed in germinal center centroblasts, is indispensable for this process. Expression of AID is sufficient to activate SHM in hybridomas, non-B cells, and Escherichia coli, suggesting that it initiates the mutational process by deaminating DNA. However, the cis-acting sequences that are responsible for targeting AID activity to the variable (V) region of Ig genes are unknown. Here we show that expression of AID in B cell lines (i.e., Burkitt's lymphoma Ramos and hybridoma P1-5) not only causes hypermutation of Ig sequences, but also of the AID transgene itself. Because it is possible that B cell-specific transacting factors bind to and recruit the "mutator" to the AID transgene, we tested whether the AID transgene can mutate in non-B cells. Indeed, we show that expression of AID in Chinese hamster ovary cells causes SHM of both the Ig and AID transgenes. These data suggest that high transcription rates alone may predispose any gene to mutation by AID but do not preclude that there are specific B cell factors that account for the extremely high rate of V mutation in vivo and its targeting to the V region.