The Aicda gene encodes Activation-Induced cytidine Deaminase (AID), an enzyme essential for remodeling antibody genes in mature B lymphocytes. AID is also responsible for DNA damage at oncogenes, leading to their mutation and cancer-associated chromosome translocation in lymphoma. We used fate mapping and AID^GFP reporter mice to determine if AID expression in the mouse extends beyond lymphocytes. We discovered that AID^cre tags a small fraction of non-lymphoid cells starting at 10.5 days post conception (dpc), and that AID^GFP+ cells are detectable at dpc 11.5 and 12.5. Embryonic cells are tagged by AID^cre in the submandibular region, where conditional deletion of the tumor suppressor PTEN causes squamous papillomas. AID^cre also tags non-lymphoid cells in the embryonic central nervous system. Finally, in the adult mouse brain, AID^cre marks a small fraction of diverse neurons and distinct neuronal populations, including pyramidal cells in cortical layer IV.