We compare the expression patterns of the three mouse Teashirt (mTsh) genes during development of the forebrain and at a postnatal stage. During development, mTsh genes are expressed in domains that are restricted both dorsoventrally and rostrocaudally, with major changes in expression level coinciding with compartment boundaries. Striking complementarities in the distribution of mTsh transcripts were observed in the developing diencephalon, telencephalon, and olfactory bulb (OB). A mTsh1‐positive cell population is part of the DLX‐positive population localized in the dorsalmost portion of the lateral ganglionic eminence (dLGE). Comparison of the mTsh1 expression domain with the domains of Er81 and Islet1, which mark two distinct progenitor populations in the subventricular zone of the LGE, suggests that mTsh1 marks OB interneuron progenitors. Furthermore, the distinct expression patterns of mTsh1 and mTsh2 in the ventral LGE and the dLGE highlight the differential contributions of these structures to the striatum and the amydaloid complex. For Sey/Sey mutants, we show that Pax6 function is critical for the correct specification of the mTsh1⁺ population in the dLGE during embryogenesis. At postnatal stages in the OB, mTsh1 is expressed in granule and periglomerular cells, which originate from the subpallium during development. Furthermore, mTsh1⁺ cells line the walls of the anterior lateral ventricle, a region that gives rise to the interneurons that migrate in the rostral migratory streams and populate the OB postnatally. Our results suggest a role for mTsh genes in the establishment of regional identity and specification of cell types in the developing and adult forebrain. J. Comp. Neurol. 486:76–88, 2005. © 2005 Wiley‐Liss, Inc.