Protein containing one or more covalently linked glucose residues, resulting from a non-enzymatic spontaneous reaction. The carbohydrate is attached to an amino-acid nitrogen atom (e.g. from a lysine side chain, or the amino-terminal group). This modification is a side effect of diabetes and aging. Glycation is the first step toward the formation of advanced glycation endproducts (AGEs). Some AGEs are benign, but others are implicated in age-related chronic diseases such as: type II diabetes mellitus, cardiovascular diseases, Alzheimer's disease, etc.