Arginine methylation is a common posttranslational modification that is found on both histone and non-histone proteins. Three types of arginine methylation exist in mammalian cells: monomethylarginine (MMA), asymmetric dimethylarginine (ADMA) and symmetric dimethylarginine (SDMA). The most prevalent is omega-NG,NG-dimethylarginine. Here, two methyl groups are placed on one of the terminal nitrogen atoms of the guanidino group; this derivative is commonly referred to as asymmetric dimethylarginine (ADMA). Two other derivatives occur at levels of about 20% to 50% that of ADMA. These include the symmetric dimethylated derivative, where one methyl group is placed on each of the terminal guanidino nitrogens and the monomethylated derivative with a single methyl group on the terminal nitrogen atom . These three derivatives are present on a multitude of distinct protein species in the cytoplasm, nucleus, and organelles of mammalian cells. Methylated arginine residues in proteins are often flanked by one or more glycine residues, but there are many exceptions to this general rule.