Although the underlying mechanism for this iron infusion-induced FGF23-related hypophosphatemia remains unclear, it has already been clarified that the final rate-limiting step to determine the serum intact FGF23 level is the posttranslational O-glycosylation initiated by N-acetylgalactosaminyltransferase 3 (GalNAc-T3) (Frishberg et al., 2007; Takashi et al., 2019). The gene discussed is CHSY3; the disease is hypophosphatemia.