B3GALTL is a beta-1,3-glucosyltransferase that transfers glucose to O-linked fucosylglycans on thrombospondin type-1 repeats (TSRs) of several proteins. B3GALTL is a type II membrane protein. Defects in B3GALTL gene are a cause of Peters-plus syndrome (PPS). As an O-fucosyltransferase, B3GALTL transfers glucose toward fucose with a beta-1,3 linkage. It specifically glucosylates O-linked fucosylglycan on TSP type-1 domains of proteins, thereby contributing to elongation of O-fucosylglycan.