Figure 1: Classes of protein domain or peptide tags used for chemical labeling of fusion proteins. Legend: A: domain tags self-label either covalently or noncovalently. B: peptide tags either self-label with varying degrees of reversibility or utilize enzyme-mediated covalent attachment. The standard methods of fluorescent protein or epitope tags are shown for comparison. Examples of tags of each class are listed to the right. Lin MZ and Wang L. Selective Labeling of Proteins with Chemical Probes in Living Cells. Physiology 23: 131-141, 2008.