Cell sorting was performed on a MoFlo triple laser instrument (DakoCytomation, Fort Collins, CO) using Summit 3.1 software (Dako Cytomation). Analysis of raw data was completed with FlowJo software (Treestar, Ashland, OR). The laser emissions were 488, 350, and 647 nm. Fluorescence was detected with the following bandpass filters: 530/40 for FITC and GFP, 580/30 for PE, 670/30 for PI, 405/30 for Hoechst blue, 570/20 for Hoechst red, and 670/20 for APC (Omega Optical, Brattleboro, VT). Hoechst blue and red fluorescence were detected in linear-scale acquisition. First, a live cell gate was created excluding cell fragments (low forward scatter) or events that contained high PI fluorescence. Cells within this live cell gate were displayed on a Hoechst-red–Hoechst-blue histogram, and side population (SP) cells were identified after collecting at least 1 × 10⁵ events (gating algorithm illustrated in Figure S1, available on the Blood website; see the Supplemental Materials link at the top of the online article).