Jansen metaphyseal chondrodysplasia (JMC) is an ultra-rare autosomal dominant disease that is caused by heterozygous, activating PTH1R mutations resulting in PTH- and PTHrP-independent hypercalcemia and hypercalciuria, leading to nephrocalcinosis and impaired renal function later in life. The gene discussed is PTHLH; the disease is Hypercalcemia.