Based on limited published evidence, PGAP4 is a Golgi-resident glycosyltransferase that catalyzes N-acetylgalactosamine (GalNAc) transfer to glycosylphosphatidylinositol (GPI)-anchored proteins 1. The enzyme transfers GalNAc from UDP-GalNAc to the 4-OH position of the first mannose residue in the GPI anchor, occurring after fatty acid remodeling during GPI maturation 1. Structurally, PGAP4 uniquely contains three transmembrane domains with a tandem transmembrane insertion in its glycosyltransferase-A fold, distinguishing it from typical Golgi glycosyltransferases 1.