Touzet L, et al. - Role of CD9 and its association with biological factors in a cohort of 112 acute myeloid leukemia (AML) patients treated with intensive chemotherapy were investigated. Forty percent of AML expressed CD9. Univariate and multivariate analyses suggested it be associated with a favorable outcome (event-free survival and relapse-free survival). Interestingly, differences were noted in CD9 expression between the more immature physiologic and AML cells (CD34+CD38−) as CD9 was also expressed in AML on putative leukemic stem cells (LSCs) but not on hematopoietic stem cells (HSCs). This suggest the possible utility of CD9 as a very important marker for minimal residual disease (MRD) monitoring in AML based on LSC targeting.