ALPL and hypophosphatasia: Hypophosphatasia (HPP) is a rare inborn error of metabolism due to a loss-of-function mutation in the gene for tissue nonspecific isoenzyme of alkaline phosphatase (TNSALP) that results in low levels of ALP.1,2 Over 300 mutations have been reported in the TNSALP gene, which is mostly expressed in the liver, skeleton, and developing teeth.