This clinical trial is being conducted to study hypophosphatasia (HPP), a bone disorder caused by gene mutations or changes. These gene mutations cause low levels of an enzyme needed to harden bone. The purpose of this study is to test the safety and efficacy of a study drug called asfotase alfa (human recombinant tissue non-specific alkaline phosphate fusion protein) to see what effects it has on patients ≤ 5 years of age or less with HPP.