Discoidin domain receptors (DDRs) are unusual receptor tyrosine kinases (RTKs) that are activated by fibrillar collagens instead of soluble growth factors. DDRs play an important role in various cellular functions and disease processes, including malignant progression. Compared to other RTKs, DDRs have relatively long juxtamembrane domains, which are believed to contribute to receptor function. Despite this possibility, the function and mechanism of the juxtamembrane domain of DDRs have not yet been fully elucidated. In this study, we found that the cytoplasmic juxtamembrane 2 (JM2) region of DDR2 contributed to receptor dimerization, which is critical for receptor activation in response to collagen stimulation. A collagenbinding assay showed that JM2 was required for efficient binding of collagen to the discoidin (DS) domain. Immunohistochemical analysis of DDR2 expression using a tissue microarray demonstrated that DDR2 was overexpressed in several carcinoma tissues, including bladder, testis, lung, kidney, prostate and stomach. In H1299 cells, inhibition of DDR2 activity by overexpressing the juxtamembrane domain containing JM2 suppressed collagen-induced colony formation, cell proliferation and invasion via the inhibition of matrix metalloproteinase-2 and matrix metalloproteinase-9. Taken together, our results suggest that JM2-mediated dimerization is likely to be essential for DDR2 activation and cancer progression. Thus, inhibition of DDR2 function using a JM2-containing peptide might be a useful strategy for the treatment of DDR2-positive cancers.Cell surface receptors sense and respond to diverse extracellular signals, which in turn initiate appropriate cellular responses. Receptor tyrosine kinases (RTKs) constitute a major class of cell surface receptors with high affinities for various polypeptide growth factors, cytokines and hormones. Hence, RTKs have been implicated in normal physiology, and altered or abnormal RTK regulation has been implicated in pathology, including cancer progression. [1][2][3] In the classical model of RTK activation, receptors exist as monomers in the absence of a ligand and dimerize on ligand binding. 4 This dimerization brings the cytoplasmic domains into close proximity, leading to transphosphorylation. 5 However, several studies have shown that receptors for cytokines and growth factors exist in preformed dimeric or trimeric states in the absence of a ligand through their transmembrane (TM) domains or through extracellular juxtamembrane (EJM) and intracellular juxtamembrane (IJM) regions, and these receptors undergo conformational changes in the catalytic domain in response to ligand stimulation. [6][7][8] It was reported that the IJM region of several RTKs contained phosphorylated Tyr residues, which are used as binding sites for proteins involved in downstream signaling, indicating that IJM has a functional role in RTK activation. 9 For example, the IJM of epidermal growth factor receptor (EGFR) has an autoactivating role. 10 However, the precise molecular mechanism has ...