ALPL and hypophosphatasia: We classified patients as mild HPP if they fulfilled all of the following criteria: (1) persistently low ALP (at least 2 measurements ≤35 U/L); (2) heterozygous or compound heterozygous for a pathogenic or likely pathogenic variant in ALPL, following the ACGM guidelines; (3) at least two of the following symptoms: (A) history of a low energy fracture, (B) premature loss of baby or permanent teeth, (C) chronic musculoskeletal pain, (D) muscle weakness.