CD9 and acute promyelocytic leukemia: We also found that normal promyelocytes express lower levels of CD9 than leukemic blasts of APL, supporting the findings of Wen et al14 that it can be used as another marker to help define abnormality in MRD testing.14 Normal HSCs express slightly higher levels of CD9 than CMPs, with increased and/or atypical expression patterns potentially being useful to define abnormality in AML posttherapy, although further studies to confirm these findings will be necessary.