In the anterior pituitary gland, five distinct hormone producing cell types are generated from common epithelial progenitor cells. This process of cellular determination is controlled by signaling molecules acting on transcription factors, which in turn regulate genes critical for the appearance of hormone producing cells. Two pituitary specific transcription factors, Pit-1 and Prop-1, which are required for generation of hormone producing cells in the anterior pituitary gland, have proved to be important in the cause of hypopituitarism in humans. Mutations in the human PIT-1 gene lead to a combined pituitary hormone deficiency characterized by lack of GH, PRL and TSH. Mutations in the PROP-1 gene, which are a more common cause of hypopituitarism, lead to a clinical phenotype characterized by GH, PRL, TSH, LH and FSH deficiency, and sometimes ACTH deficiency as well.