MSI2 and neoplasm: To determine whether Msi2 expression marks cells with increased capacity for tumor growth, we generated Msi2-reporter KrasG12D/+; Trp53fl/fl mice with inducible Cre expression (Msi2eGFP/+; KrasG12D/+; Trp53fl/fl; Rosa26CreERT2/+; Figure 2A), delivered tamoxifen, isolated Msi2+ and Msi2− lung epithelial cells, and tested sphere-forming capacity.