MSCs were transfected with a pCVSV-G-1 plasmid [45] that encodes VSV-G under a CAG promoter using the Neon Transfection System (Invitrogen), according to manufacturer's protocol. Briefly, 5 × 10 5 cells were transfected with 2 μg of plasmid with one 1,300 V pulse for 20 msec and plated in 6-well plates. To determine transfection efficiency, electroporated cells were cultured for 24 h and immunocytochemistry (ICC) was performed to detect VSV-G protein expression. Briefly, cells were washed with two rinses and two incubations of 1X PBS. Cell fixation was performed with 4% PFA, followed by another set of washes, and probed with the 1 : 50 dilution of FITCconjugated anti-VSV-G antibody (GeneTex, San Antonio TX, USA) in 3% BSA for 60 min. Cultures were washed a final time and mounted in DABCO/DAPI mounting medium (2. The Quantum MESF kit consists of 5 populations of microspheres with increasing surface-labeled fluorochrome, which have been standardized to specific concentrations of pure fluorophore per microsphere. Each population was analyzed via flow cytometry and a standard curve was generated by plotting population (i.e., concentration of fluorophore per microsphere) versus intensity. QuickCal software was used to verify the linearity of the standard curve. Next, vMSCs and corresponding control populations were labeled with an anti-VSV-G-FITC antibody and analyzed via flow cytometry. Using the standard curve and the measured intensity value for vMSC populations and corresponding controls, the number of fluorophores per cell was determined. This value was divided by the average number of fluorophores (4.2) that bind to a single antigen to determine the number of proteins expressed per cell. Ten thousand cells and three replicates were analyzed per population. Populations included vMSCs with anti-VSV-G antibody, MSCs with anti-VSV-G antibody and vMSCs without antibody.