Stromal cell–conditioned media promotes SSA in imatinib-treated K562 cells. K562 cells were cotransfected with SAGFP and the I-SceI expression vector. SSA frequency was determined by flow cytometry after 48 hours in the presence of imatinib (1 μM) and stromal cell–conditioned media derived from KM104, KM105, HS5, or HS27A, as indicated (n = 3).