Prophet of PIT1 (PROP1) is a pituitary-specific factor and responsive gene for the combined pituitary hormone deficiency in Ames dwarf mice and human patients. Our immunohistochemical studies demonstrated that PROP1 is consistently expressed in SOX2-expressing stem/progenitor cells in the rat pituitary from embryonic (E) to postnatal periods. At E13.5, all the cells in Rathke's pouch, the primordium of the pituitary, express PROP1. Afterward, PROP1-positive cells localize along the marginal cell layer, a putative stem cell niche in the pituitary, and stratify in the parenchyma of the anterior pituitary. In the embryonic period, PROP1 coexists transiently with PIT1, which is the anterior pituitary-specific factor and is a target of PROP1, but not any hormones. Thus, the present results imply a regulatory role of PROM not only in pituitary organogenesis but also in conversion of PIT1-lineage cells. (C) 2009 Elsevier Inc. All rights reserved.