Dogma that the regulatory T cell (Treg) prevents catastrophic autoimmunity throughout the lifespan relies on the assumption that the FoxP3 locus is transcribed exclusively in Treg. To test the assumption, we used the Rag2(-/-) and the Rag2(-/-) mice with the Scurfy (sf) mutation (FoxP3(sf/Y) or FoxP3(sf/sf)) to evaluate FoxP3 expression outside of the lymphoid system. Immunohistochemistry and real-time PCR revealed FoxP3 expression in breast epithelial cells, lung respiratory epithelial cells, and prostate epithelial cells, although not in liver, heart, and intestine. The specificity of the assays was confirmed, as the signals were ablated by the Scurfy mutation of the FoxP3 gene. Using mice with a green fluorescence protein open reading frame knocked into the 3' untranslated region of the FoxP3 locus, we showed that the locus is transcribed broadly in epithelial cells of multiple organs. These results refute an essential underlying assumption of the dogma and question the specificity of FoxP3-based Treg depletion.