Although genomic hybridization and loss of heterozygosity (LOH) studies have suggested the presence of an undiscovered parathyroid tumor suppressor gene on the X chromosome [11, 24, 25], and notwithstanding that ZFX deletion has led to tumorigenesis in an experimental liver cancer model in mice [26], it does not appear that ZFX is acting as a classic two-hit human tumor suppressor in parathyroid adenomas. The gene discussed is ZFX; the disease is neoplasm.