as well as several mannose residues, depending on the N-linked glycan in question. The removal of the glucose residues is dependent on proper protein folding. These processing reactions occur in the Golgi apparatus. Modification reactions may involve the addition of a phosphate or acetyl group onto the sugars, or the addition of new sugars, such as neuraminic acid. Processing and modification of N-linked glycans within the Golgi does not follow a linear pathway. As a result, many different variations of N-linked glycan structure are possible, depending on enzyme activity in the Golgi. Functions and importance N-linked glycans are extremely important in proper protein folding in eukaryotic cells. Chaperone proteins in the endoplasmic reticulum, such as calnexin and calreticulin, bind to the three glucose residues present on the core N-linked glycan. These chaperone proteins then serve to aid in the folding of the protein that the glycan is attached to. Following proper folding, the three glucose residues are removed, and the glycan moves on to further processing reactions. If the protein fails to fold properly, the three glucose residues are reattached, allowing the protein to re-associate with the chaperones. This cycle may repeat several times until a protein reaches its proper conformation. If a protein repeatedly fails to properly fold, it is excreted from the endoplasmic reticulum and degraded by cytoplasmic proteases. N-linked glycans also contribute to protein folding by steric effects. For example, cysteine residues in the peptide may be temporarily blocked from forming disulfide bonds with other cysteine residues, due to the size of a nearby glycan. Therefore, the presence of a N-linked glycan allows the cell to control which cysteine residues will form disulfide bonds. N-linked glycans also play an important role in cell-cell interactions. For example, tumour cells make N-linked glycans that are abnormal. These are recognized by the CD337 receptor on Natural Killer cells as a sign that the cell in question is cancerous. Within the immune system the N-linked glycans on an immune cell's surface will help dictate that migration pattern of the cell, e.g. immune cells that migrate to the skin have specific glycosylations that favor homing to that site. The glycosylation patterns on the various immunoglobulins including IgE, IgM, IgD, IgE, IgA, and IgG bestow them with unique effector functions by altering their affinities for Fc and other immune receptors. Glycans may also be involved in "self" and "non self" discrimination, which may