FHPT may also present as non-syndromic PHPT, which includes familial hypocalciuric hypercalcemia (FHH) forms (caused by variants of CASR, GNA11, or AP2S1), neonatal severe hyperparathyroidism, and other forms caused by several genes classified as familial isolated hyperparathyroidism (FIHP). Here, AP2S1 is linked to familial isolated hyperparathyroidism.