Inheritance can be autosomal recessive or dominant, and penetrance is variable resulting in a wide range of clinical expressivity, with a spectrum ranging from stillbirth without mineralized bone to early loss of teeth without bone symptoms. Depending on the age at diagnosis six clinical forms are currently recognized: perinatal (lethal), perinatal benign, infantile, childhood, adult and odontohypophosphatasia. Severe forms of HPP (perinatal and infantile) are inherited as an autosomal recessive trait and in milder forms (adult and odontohypophosphatasia) autosomal recessive and autosomal dominant inheritance coexist.