Transcription factor involved in the specification of the lactotrope, somatotrope, and thyrotrope phenotypes in the developing anterior pituitary. Activates growth hormone and prolactin genes. Specifically binds to the consensus sequence 5'-TAAAT-3'. Interacts with PITX1. Interacts with LHX3. Interacts with ELK1. The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. Defects in Pou1f1 are the cause of the dwarf (dw) phenotype which interrupts the normal development of the anterior pituitary gland, resulting in the loss of expression of growth hormone, prolactin and thyroid-stimulating hormone, and hypoplasia of their respective cell types. Unable to transactivate. Only about 1/7 as abundant as isoform 1. Belongs to the POU transcription factor family. Class-1 subfamily.