Figure 10.25 The formation of a mannose 6-phosphate marker. A glycoprotein destined for delivery to lysosomes acquires a phosphate marker in the Golgi complex in a two-step process. First, N-acetylgalactosamine phosphotransferase adds a phospho-N-acetylglucosamine unit to the 6-OH group of a mannose residue; then, N-acetylgalactosaminidase removes the added sugar to generate a mannose 6-phosphate residue in the core oligosaccharide.