The tissue-specific POU domain transcription factor Pit-1 is required for phenotypic specification of three cell types in the anterior pituitary gland of mammals: somatotropes, thyrotropes, and lactotropes. Mutations in thePit-1gene, as first described in Snell and Jackson dwarf (dw) mice, led to pituitary hypoplasia due to lack of these three cell types, as well as hypothyroidism and dwarfism because of deficiency of thyroid-stimulating hormone and growth hormone, respectively. The Ames dwarf (df) exhibits a phenotype identical to that of Pit-1-mutated mice but the defective gene has been mapped to a locus on mouse chromosome 11, distinct from thePit-1gene located on mouse chromosome 6. Our studies indicate that initial activation of thePit-1gene is deficient in the Ames dwarf. This suggests that thedfgene is required for activation of thePit-1gene and provides evidence for a hierarchy of tissue-specific factors required for cellular commitment in the anterior pituitary gland.