The function of POFUT3 (protein O-fucosyltransferase 3, Ensembl gene identifier ENSG00000172728) is as follows. Protein O-fucosyltransferase that specifically catalyzes O- fucosylation of serine or threonine residues in EMI domains of target proteins, such as MMRN1, MMRN2 and EMID1 (PubMed:39775168). Attaches fucose through an O-glycosidic linkage (PubMed:39775168). O- fucosylation of EMI domain-containing proteins may be required for facilitating protein folding and secretion (PubMed:39775168). May also show alpha-(1,3)-fucosyltransferase activity toward the innermost N- acetyl glucosamine (GlcNAc) residue in biantennary N-glycan acceptors (PubMed:19088067). However, this was tested with a library of synthetic substrates and this activity is unsure in vivo (PubMed:19088067). May be involved in biosynthesis of Lewis X-carrying biantennary N-glycans that regulate neuron stem cell self-renewal during brain development (By similarity). {ECO:0000250|UniProtKB:Q5F2L2, ECO:0000269|PubMed:19088067, ECO:0000269|PubMed:39775168}.