POMC and ACTH-producing pituitary gland adenoma: However, the 2017 World Health Organization (WHO) classification of PAs redefined a corticotroph adenoma as a PA that expresses ACTH and other proopiomelanocortin (POMC)–derived peptides and arises from adenohypophyseal cells of pituitary-restricted transcription factor (Tpit) lineage (1).