A fusion protein of a MAb against the human TfR, and iduronate 2-sulfatase (IDS), the lysosomal enzyme mutated in MPSII, and designated pabinafusp alfa, was administered chronically by weekly IV infusion to pediatric patients with MPSII, and this treatment stabilized cognitive function and reduced CSF glycosaminoglycan, leading to the first market approval of a BBB-penetrating IgG-biologic fusion protein in Japan (Sonoda et al., 2022). Here, TFRC is linked to mucopolysaccharidosis type 2.