CD9 and acute lymphoblastic leukemia: In our study, CD9 is expressed in 40% of AML and, considering its expression at a significant level on LSCs, it should be investigated when these cells are studied by the MRD strategy that we described previously.11 In ALL, CD9 seems to be associated with cancer stem cell properties too and is involved in leukemic progression.25 Interestingly, CD9 antigen level of expression is identical between LSCs and AML blast cells and is not expressed on normal HSCs.