MBD4 protein binds preferentially to fully methylated CpG sites. Two of DNA's four bases, cytosine and adenine, can be methylated. To produce carnitine, lysine residues are methylated to trimethyllysine. Furthermore, -DOPA also can be methylated in the methylation process to 3-O-methyldopa. One example of a protein which is commonly methylated is a histone. Therefore, DNA that has been treated with bisulfite retains only methylated cytosines. However, 2,568 CpGs among 942 genes remained differentially methylated in former versus never smokers. Thus, trivalent inorganic arsenic compounds are methylated to give methanearsonate. Lysine can be methylated once, twice, or three times by lysine methyltransferases. When many of a gene's promoter CpG sites are methylated the gene becomes silenced. The ribose of the adjacent nucleotide may also be methylated to give a cap 1. The targeted site is methylated, epigenetically modifying the gene. The MeCP2 protein binds to forms of DNA that have been methylated. Multiple sites of proteins can be methylated. For example, DNA may be methylated by S-adenosylmethionine, and glycosylated by reducing sugars.