Genes encoding novel proteins named AID (Activation-Induced cytidine Deaminase) are described. The AID proteins are structurally related to APOBEC-1, an RNA editing enzyme, and have a cytidine deaminase activity similar to APOBEC-1. The AID genes were found by preparing cDNA libraries from mouse B cell clone CH12F3-2 (which undergoes class switch recombination from IgM to IgA at an extremely high rate after activation of the cells by stimulation with cytokines), with and without stimulation with cytokines, and performing subtraction cloning using the libranes.