CD9 and neoplasm: Although many molecules, including CD44v6 [17], c-Met [18], urokinase-type plasminogen activator receptor [19], CD97 [7], CD9 [20], CD151 [21], CD11b [14], D6.1A [20], CD13 [22], CD49 [23], CD104 [24], EpCam [25], and claudin 7 [26], participate in this process, the recruitment incentive of these molecules to the metastatic site and the mechanism of long-distance communication with the local tumor remain unknown.