PTHLH and Hypercalcemia: Jansen's metaphyseal chondrodysplasia (JMC) is an ultra-rare autosomal dominant disease that is caused by heterozygous, activating PTH1R mutations (4-6) resulting in PTH- and PTHrP-independent hypercalcemia and hypercalciuria, leading to nephrocalcinosis and impaired renal function later in life (6, 7).