MEN1 is caused by germline <i>MEN1</i> mutations in > 75% of patients, and the remaining 25% of patients may have mutations in unidentified genes or represent phenocopies with mutations in genes such as cell cycle division 73 (<i>CDC73)</i>, the calcium sensing receptor (<i>CASR)</i>, and cyclin-dependent kinase inhibitor 1B (<i>CDKN1B)</i>, which are associated with the hyperparathyroidism-jaw tumor syndrome, familial hypocalciuric hypercalcemia type 1, and MEN4, respectively. This evidence concerns the gene CDKN1B and hyperparathyroidism.