CASR and primary hyperparathyroidism: In addition, children inheriting a monoallelic loss-of-function CaSR mutation from the father are at risk of neonatal severe primary hyperparathyroidism if the mother is normocalcemic, whereas they would likely develop the more benign phenotype of FHH1 if they inherited the loss-of-function CaSR mutation from the mother (5), and can occasionally be normocalcemic because of incomplete penetrance of the CaSR mutation (6).