ALPL and hypophosphatasia: Hypophosphatasia (HPP), also known as Rathbun disease, is a rare, heritable inborn error of metabolism that has both autosomal dominant and recessive inheritance patterns due to loss‐of‐function mutations in the alkaline phosphatase gene (ALPL) located on the short arm of chromosome 1, which encodes the tissue nonspecific isozyme of alkaline phosphatase (TNSALP).1, 2, 3, 4 Clinical features of HPP include low serum alkaline phosphatase (ALP) activity, defective mineralization of bone, and elevated levels of ALP substrates such as pyridoxal‐5‐phosphate (vitamin B6).