The biological activity of peptides is regulated through their binding to corresponding receptors. Peptides that target receptors overexpressed on tumor cells\u2014but not on normal cells\u2014are excellent candidates for in vivo<\/em> tumor imaging. To date, many peptides and their analogs have been identified for disease detection. Some representative peptides are summarized below:<\/p>\n\n\n\n 1. RGD<\/p>\n\n\n\n The tripeptide RGD can specifically bind to integrin receptors. Integrins are composed of two subunits (\u03b1 and \u03b2). The integrin family, especially \u03b1V\u03b23, plays a crucial role in tumor angiogenesis and metastasis. During angiogenesis, these receptors are overexpressed on endothelial cells but are barely detectable in most normal organs. Therefore, they are widely used in diagnostic imaging.<\/p>\n\n\n\n Single-letter code: H2N-RGD-OH<\/p>\n\n\n\n Three-letter code: H2N-Arg-Gly-Asp-OH<\/p>\n\n\n\n Number of amino acids: 3<\/p>\n\n\n\n Molecular formula: C12H22N6O6<\/p>\n\n\n\n Average molecular weight (MW): 346.34<\/p>\n\n\n\n RGD Peptides<\/a><\/p>\n\n\n\n