Recent molecular pathological investigations have shown the multihormonal expression in somatotroph adenomas and clinically non-functioning adenomas. There are also some reports on somatotroph adenomas showing hypersecretion of adrenocorticotropic hormone. The tumorigenesis of isolated pituitary adenomas has not been identified, nevertheless, the monoclonality of pituitary adenomas has been demonstrated by several investigators; pituitary adenomas originate monoclonally from a single adenohypophyseal cell with subcellular abnormalities. The cytodifferentiation of pituitary cells has also recently elucidated extensively, and lots of transcriptional factors are found to regulate the cytodifferentiation of pituitary cells. This scheme of cytodifferentiation is closely related to the pathogenesis of pituitary adenomas. This review article focuses on the receptor of pituitary adenomas and the function of transcriptional factors regulating the cytodifferentiation of pituitary cells.