After FACS sorting, LW12-TetOn3G-APOL1/WT/Vs harboring HMSCs were cultured in SMCM to around 80–90% confluence, and then were changed to RPMI medium containing 10% FBS and 100 ng/ml doxycycline. After another 4 days, the medium was collected, and free-floating cells were cleared by centrifugation. APOL1 concentrations in the collected CMs were determined by ELISA (A). The CMs were used to treat differentiated human podocytes in cell culture dishes, and the cells were subjected to Trypan Blue staining to count the alive cells (B). * p < 0.05 compared with vector, while # p < 0.05 compared to APOL1WT.