PROP1 is expressed in the pituitary gland, and plays an important role in the morphogenesis of the pituitary gonadotropes as well as lactotropes, somatotropes and caudomedial thyrotropes. It also controls the expres­sion of growth hormone (GH) prolactin (PRL) and thyroid stimulating hormone (TSH) subunits, through regulatory PIT1 factor. STAT5A was discovered ini­tially as a PRL-induced transcription factor. It is a key intracellular mediator of prolactin signaling and can activate transcription of milk protein genes in response to prolactin.