HPP is an inborn error of metabolism characterised biochemically by low serum alkaline phosphatase (ALP) activity (hypophosphatemia) and caused by a loss-of-function mutation within the gene encoding tissue-nonspecific alkaline phosphatase (TNAP) (ALPL; OMIM#171760) on chromosome 1p36 [86-88]. Here, ALPL is linked to hypophosphatemia.