The underlying mechanisms may involve the following aspects: i) The interaction between CD9+ cancer cells and ECs is important in transendothelial migration of the tumor cells; ii) CD9 is likely to enhance the motility, such as invasion, in several types of cancer; iii) CD9 promotes tumor cell growth and prevents apoptosis from chemotherapy drugs; and iv) CD9 is becoming an increasingly important indicator for the identification of CSCs, since CSCs are crucial in oncogenesis. Here, CD9 is linked to cancer.