Msi1 expression in breast cancer tissue microarrays. A. Upper panel, Msi1 is rarely expressed in large ductal epithelial cells of normal breast tissue (red arrow), Lower panel, Msi1 is frequently expressed in terminal ductal myoepithelial cells (black arrows), whereas, one Msi1+ cell is present in terminal ductal epithelial cells (red arrow). B. Msi1 and smooth muscle actin (SMA) are co-expressed in terminal ductal myoepithelial cells. C. Upper left panel, a breast cancer specimen "negative" for Msi1 expression (<1% positive cells). Upper right panel, a breast cancer specimen with "low" Msi1 expression (1-30% positive cells). Lower Left panel, a breast cancer specimen with "high" Msi1 expression (>31% positive cells). Lower right panel, the relative percentages of "negative", "low" and "high" Msi1 expression in all samples (N = 140). D. "Low" and "high" Msi1 expression in lymph node metastases. E. Msi1 is expressed in all metastatic breast cancer specimens. Bar graph indicates that 84% of lymph node metastases (LN; N = 19) had high Msi1 expression vs. 42% of matched primary tumors (Primary BC; N = 19) and 52% of all breast cancer specimens (Total BC; N = 140). F. Kaplan-Meier analysis indicates that a larger percentage of patients with Msi1+ tumors (N = 47) survive <5 years vs. patients with Msi1- tumors (N = 28). These results suggest that Msi1 is a negative prognostic marker.