The current consensus is that CD9 is a metastasis suppressor. CD9 expression has been inversely associated with the metastasis of cancers. That is low CD9 protein expression is associated with tumour samples taken from secondary metastatic sites compared to the primary tumour site. Furthermore in a number of cancer cell lines and primary cells, CD9 expression has been shown to decrease cell motility. A number of studies however conversely demonstrate that CD9 can increase the motility of cancer and “normal” cells. This does not fit with the metastasis suppressor hypothesis and has not been adequately explained by current models of CD9 function during metastasis.
These apparently contradictory observations may stem from the large number of functionally distinct steps that the term metastasis covers. … Furthermore recent evidence suggests that contrary to the traditional view that metastasis occurs late in cancer progression, metastasis may occur early in relatively normal cells. This has serious implications for the role of CD9 and many other proteins in metastasis. CD9 is widely expressed in many normal cells. If metastasis occurs early this suggests that CD9 expression does not inhibit metastasis.