This classification is rooted in 2 key observations from the registry, which have broadened our understanding of HPP: (1) that the disease in patients who present before 6 months of age is biologically different from the disease in patients who present after 6 months of age based on the presence of life-threatening manifestations and greater likelihood of having 2 ALPL variants and (2) that there are few differences in overall disease burden among patients who present after 6 months of age [13, 40, 43, 48, 49, 53]. The gene discussed is ALPL; the disease is hypophosphatasia.