AP2S1 and familial hypocalciuric hypercalcemia: 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).