Hypophosphatasia is an inborn error of metabolism that is characterized clinically by defective bone mineralization and biochemically by deficient activity of the tissue-nonspecific isoenzyme of alkaline phosphatase (TNSALP) in serum and in tissues. Clinical severity is extremely variable, ranging from death in utero to pathologic fractures first presenting in adulthood. Severe forms of the disease are inherited in an autosomal recessive fashion; the modes of transmission of mild forms are uncertain. Deficiency of TNSALP activity in this condition suggests that mutations in the TNSALP "candidate" gene are the primary defects. This hypothesis was supported in 1988 by the demonstration, in one inbred infant, that an identical missense mutation in both alleles of the gene encoding TNSALP caused lethal hypophosphatasia. Here we summarize the work leading to that discovery and discuss the recent identification of additional missense mutations in the TNSALP gene associated with the entire clinical spectrum of hypophosphatasia.