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. Altered in its ability to trans-activate compared to isoform B. Belongs to the POU transcription factor family. Class-1 subfamily.