Transcription factor involved in the specification of the lactotrope, somatotrope, and thyrotrope phenotypes in the developing anterior pituitary. Specifically binds to the consensus sequence 5'-TAAAT-3'. Activates growth hormone and prolactin genes (PubMed:22010633, PubMed:26612202). Interacts with PITX1 (PubMed:26612202). Interacts with LHX3 (PubMed:26612202). Interacts with ELK1 (PubMed:26612202). The 9aaTAD motif is a transactivation domain present in a large number of yeast and animal transcription factors. The disease is caused by variants affecting the gene represented in this entry. Altered in its ability to trans-activate compared to isoform B. Belongs to the POU transcription factor family. Class-1 subfamily.