Hypophosphatasia is a rare and sometimes fatal metabolic bone disease. Clinical symptoms are heterogeneous, ranging from the rapidly fatal perinatal variant, with profound skeletal hypomineralization and respiratory compromise, to a milder, progressive osteomalacia later in life. Tissue non-specific alkaline phosphatase (TNSALP) deficiency in osteoblasts and chondrocytes impairs bone mineralization, leading to rickets or osteomalacia. The pathognomonic finding is subnormal serum activity of the TNSALP enzyme, which is caused by one of 200 genetic mutations identified to date in the gene encoding TNSALP. Genetic inheritance is autosomal recessive for the perinatal and infantile forms but either autosomal recessive or autosomal dominant in milder forms. The prevalence of hypophosphatasia is not known. One study estimated the live birth incidence of severe forms to be 1:100,000.