B cells are capable of producing a diverse repertoire of antibodies to counter various pathogens. Antibody diversity is achieved through a series of programmed DNA recombination and mutation events that take place in the immunoglobulin (Ig) genes during B cell maturation. A key factor involved in Ig diversification is Activation Induced Cytidine Deaminase (AID). AID functions by deaminating cytidines into uridines in target DNA within the Ig loci; subsequent processing of the uracils, via DNA repair and replication, leads to three distinct forms of Ig diversification: class switch recombination, somatic hypermutation, and Ig gene conversion. Although AID contributes to effective immunity, its mutagenic activity also poses a danger to the host, since cytidine deamination outside of the Ig genes could generate deleterious mutations. Thus, the function of AID must be under stringent control, and we are interested in understanding the regulatory mechanism. We addressed this issue in the chicken B cell line DT40, which undergoes AID mediated Ig gene conversion. Taking advantage of the efficient homologous recombination activity of this cell line, we performed a systematic genetic analysis of the elements and factors that could be involved in the regulation of AID function. We identified a cis-acting element that is not only important for AID mediated gene conversion at the Igl locus, but is also capable of targeting AID function to ectopic loci in the genome. We show that the function of this regulatory element is mediated by several transcription factors, and two of these transcription factors, PU.1 and IRF4, play the additional role of stimulating the expression of AID gene in B cells.   Our future study is aimed at understanding in detail how these factors and elements target AID function to the Ig locus.