Although prolactin levels <100ng/mL in the presence of a macroadenoma suggest a diagnosis of NFPA,(7) prolactin levelsper seare not always sufficient to discriminate the cause of hyperprolactinemia owing to some overlapping values.(8) Additionally, despite being rare, the "hook effect" can cause misdiagnosis between macroprolactinomas and NFPAs (very high prolactin concentrations lead to false-negative results).(9). The gene discussed is PRL; the disease is non-functioning pituitary adenoma.