CDKN1B and prolactin-producing pituitary gland adenoma: Prolactinomas are the most common pituitary adenoma subtype in MEN1, and the second most common in MEN4 and FIPA, both in AIP-positive (even as a homogenous prolactinoma family [37]) and AIP-negative FIPA, but can occur in SDHx cases as well (Fig. 3).