The accuracy of the diagnosis of pituitary tumors has improved by routine use of immunohistochemistry (IHC) for pituitary transcription factors (PIT1—Pituitary-specific positive transcription factor 1, TPIT—T-box pituitary transcription factor, SF1—Steroidogenic factor 1, GATA3, and ERα—Estrogen receptor alpha), producing hormones and other biomarkers, which is endorsed in the 5th Edition of the WHO Classification of Endocrine and Neuroendocrine Tumors. This evidence concerns the gene POU1F1 and neuroendocrine neoplasm.