In recent years, new insight into the etiopathogenesis of hereditary pituitary tumors has emerged. Several novel susceptibility genes for these tumors have been identified and among them is CDKN1B encoding the cell cycle regulator p27. Heterozygous germline mutations in CDKN1B are associated with the predisposition to multiple neuroendocrine tumors (NETs), including pituitary tumors. Patients carrying these mutations are affected by the Multiple Endocrine Neoplasia type 4 (MEN4) syndrome. In this chapter, we discuss the current state of knowledge of the function of p27; present clinical and basic findings related to the pituitary phenotype of MEN4 patients as well as the functional characterization of the CDKN1B mutations identified in these patients.