The function of EXTL3 (exostosin like glycosyltransferase 3, Ensembl gene identifier ENSG00000012232) is as follows. Glycosyltransferase which regulates the biosynthesis of heparan sulfate (HS) (PubMed:28132690, PubMed:28148688). Initiates HS synthesis by transferring the first N-acetyl-alpha-D-glucosamine (alpha-GlcNAc) residue (GlcNAcT-I activity) to the tetrasaccharide linker (GlcA-Gal-Gal-Xyl-)Ser core linker (PubMed:11390981, PubMed:35676258). May also transfer alpha-GlcNAc residues during HS elongation (GlcNAcT-II activity) (PubMed:11390981, PubMed:35676258). Lacks glucuronyl transferase II (GlcAT-II) activity (PubMed:11390981, PubMed:35676258). Important for both skeletal development and hematopoiesis, through the formation of HS proteoglycans (HSPGs) (PubMed:11390981, PubMed:22727489, PubMed:28132690, PubMed:28148688, PubMed:35676258). Through the synthesis of HS, regulates postnatal pancreatic islet maturation and insulin secretion (By similarity). {ECO:0000250|UniProtKB:Q9WVL6, ECO:0000269|PubMed:11390981, ECO:0000269|PubMed:22727489, ECO:0000269|PubMed:28132690, ECO:0000269|PubMed:28148688, ECO:0000269|PubMed:35676258}. Receptor for REG3A, REG3B and REG3G, induces the activation of downstream signaling pathways such as PI3K-AKT or RAS-RAF-MEK-ERK signaling pathway (PubMed:22727489, PubMed:27830702, PubMed:34099862). Required for the function of REG3A in regulating keratinocyte proliferation and differentiation (PubMed:22727489). Required for the inhibition of skin inflammation mediated by REG3A through the activation of PI3K-AKT-STAT3 pathway (PubMed:27830702). Required for the function of REG3A and REG3G in glucose tolerance in pancreas (PubMed:19158046). Expressed in microglia, is activated by nociceptor- derived REG3G in response to endotoxins, leading to the inhibition of kynurenine pathway to prevent endotoxic death (By similarity). {ECO:0000250|UniProtKB:Q9WVL6, ECO:0000269|PubMed:19158046, ECO:0000269|PubMed:22727489, ECO:0000269|PubMed:27830702, ECO:0000269|PubMed:34099862}.