CD9 and breast cancer: CD9, the most intensively upregulated membrane molecule, was selected as a candidate marker for AML LSCs and has been reported to be involved in several types of CSCs, including pancreatic cancer stem cells, breast cancer stem cells, ovarian cancer stem cells, glioblastoma stem cells, and LSCs in B-acute lymphoblastic leukemia [15, 20, 22, 45, 46].