Receptor Tyrosine Kinases (RTKs) are widely expressed transmembrane proteins that act as receptors for growth factors, neurotrophic factors, and other extracellular signaling molecules. Upon ligand binding, they undergo tyrosine phosphorylation at specific residues in the cytoplasmic tail. This leads to the binding of protein substrates and/or the establishment docking sites for adaptor proteins involved in RTK-mediated signal transduction. RTKs have critical functions in several developmental processes including regulating cell survival, proliferation, and motility. When unregulated, they play prominent roles in cancer formation. R&D Systems offers several different multiplex assay formats that allow the user to assess the activity of many RTKs in a single sample. Available RTK multiplex kits include bead-based multiplex kits for the Luminex platform, membrane-based antibody arrays, and microplate-based antibody arrays.