AbstractMSI2 is highly expressed in human myeloid leukemia (AML) cell lines, and high expression of MSI2 mRNA is associated with decreased survival in AML, suggesting its use as a new prognostic marker. To test this, we measured MSI2 protein level by immunohistochemistry in 120 AML patients. Most cases (70%) showed some nuclear or cytoplasmic positivity, but the percentage of positive cells was low in most cases. Despite this, MSI2 protein expression was negatively associated with outcome, particularly for patients with good cytogenetic subgroup. For practical diagnostic purposes, the strongest significance of association was seen in cases with > 1% of cells showing strong MSI2 staining, these having a very poor outcome (P < .0001). Multivariate analysis with cytogenetic category, age, white cell count, and French-American-British subtype demonstrated that nuclear MSI2 levels were independently predictive of outcome (P = .0497). These results confirm the association of MSI2 expression with outcome in AML at the protein level and demonstrate the utility of MSI2 protein as a clinical prognostic biomarker. In addition, although positive at some level in most cases, its prognostic power derived from few positive cells, supporting its role in control of normal hematopoietic stem cell function and highlighting its role in disease progression.