MKI67 and prolactin-producing pituitary gland adenoma: The expression of oestrogen receptors (ER), ERα36 and ERα66, have been investigated in 62 prolactinomas, and it was reported that low expression of ERα36 and ERα66 was associated with tumour invasion and increased Ki-67.45 Moreover, low ERα66 expression was associated with dopamine-agonist resistance and enhanced tumour size.