CYP27B1 and hyperparathyroidism: Background: Vitamin D-dependent rickets type 1A (VDDR1A) is a rare autosomal recessive disorder caused by pathogenic variants in the CYP27B1 gene, typically characterized by growth failure, rickets, leg bowing, fracture, seizures, hyperparathyroidism, hypocalcemia, high-alkaline phosphatase, high or normal 25(OH)D3, and low 1,25(OH)2D3.