FAM20B is a Golgi-localized glycosaminoglycan xylosylkinase that catalyzes 2-O-phosphorylation of xylose residues in the proteoglycan tetrasaccharide linkage region 1. This phosphorylation acts as a molecular switch regulating proteoglycan biosynthesis by enhancing galactosyl transferase II (GalT-II/B3GALT6) activity, enabling subsequent glycosaminoglycan (GAG) chain synthesis on heparan sulfate and chondroitin sulfate proteoglycans 1. FAM20B has a unique active site structure specifically recognizing the Galβ1-4Xylβ1 disaccharide within this linker region 2. Functionally, FAM20B activity must be balanced with the 2-phosphoxylose phosphatase XYLP for optimal GAG chain synthesis; both gain-of-function and loss-of-function of FAM20B reduce GAG production 3. During temporomandibular joint development, FAM20B-catalyzed GAG synthesis regulates chondrogenic and osteogenic differentiation by controlling Indian hedgehog diffusion and signaling 4. Clinically, decreased FAM20B expression associates with cartilage degradation in osteoarthritis and Kashin-Beck disease 5. FAM20B gain-of-function inhibits glioblastoma cell proliferation and migration through GAG chain reduction 3. Loss-of-function mutations in FAM20B cause proteoglycan maturation defects similar to B3GALT6 mutations in Ehlers-Danlos syndrome 1.