Glycosylphosphatiylinositol (GPI anchor ) is a glycolipid that can be attached to the C-terminus of a protein during posttranslational modification. It is composed of a hydrophobic phosphatidylinositol group linked through a carbohydrate containing linker (glucosamine and mannose glycosidically bound to the inositol residue) to the C-terminal amino acid of a mature protein. The two fatty acids within the hydrophobic phosphatidyl-inositol group anchor the protein to the cell membrane.