GALNT3 and hypotrichosis 1: FGF23 is O-glycosylated by GALNT3 to protect it from proteolytic cleavage [10], [48], and the underlying molecular mechanism causing FTC and HHS in patients with GALNT3 mutations involves defective glycosylation of FGF23 resulting in enhanced cleavage and inactivation of FGF23 [47].