In many mammalian cells, the transport of newly synthesized or externally added 
lysosomal enzymes to lysosomes is depend on their specific recognition by 
receptors for mannose 6-phosphate (Man-6-P). The physiological importance of 
this pathway was confirmed by the finding that fibroblasts from patients with 
mucolipidosis type II (ML-II ; I - cell disease) fail to phosphorylate mannose 
residues on their newly synthesized lysosomal enzymes, which results in the 
secretion of a large percentage of their acid hydrolases into the culture 
medium. However, lysosomal enzymes themselves do not contain the any consensus 
amino acid sequences for acquiring the Man-6-P recognition marker. Kornfeld et 
al revealed using cathepsin D-pepsinogen chimera proteins that 
UDP-N-acetylglucosamine: lysosomal enzyme 
N-acetylglucosamine-1-phosphotransferase recognizes not only oligosaccharides 
but also the three-dimensional structure of the lysosomal enzymes when transfers 
N-acetylglucosamine-1-phosphate to lysosomal acid hydrolases.