Msi1 is enriched in breast tumor initiating cells. A. MCF-7 and T47D cells contain a CD133+ population. Cells sorted by FACS revealed that CD133+ cells represented 1.7% and 6.2% of the total cell population in MCF-7 and T47D cells, respectively. B. Msi1 is expressed predominantly in CD133+ cells. MCF-7 and T47D cells were sorted as in A, and CD133 and Msi1 were measured by RT-PCR. C, Spheroid cultures of MCF-7 cells are enriched in Msi1. Cells were grown for 3 weeks in ultra low attachment plates in DMEM media without serum and supplemented with growth factors. Spheroid colonies were harvested and Msi1 expression was determined by western blotting.