MSCs were induced to express VSV-G via transfection by electroporation. Low transfection efficiency would limit the ability of VSV-G to promote fusion and so VSV-G expression on MSCs was determined following electroporation. Twenty-four hours after transfection control MSCs and MSCs transfected with VSV-G (vMSCs) were probed with an anti-VSV-G antibody conjugated to fluorescein isothiocyanate (FITC) and visualized with fluorescence microscopy to determine the percentage of cells expressing VSV-G. The average transfection efficiency was 32% ± 5% (n = at least 6 optical fields per sample per trial for 3 trials, Figure 1(a) ). Since vMSCs will be harvested for in vivo studies, we also assessed VSV-G expression via flow cytometry after removal from culture plates with trypsin. We found expression of VSV-G plummeted to 5% ± 2% (n = 1 replicate per sample per trial for 3 trials, Figure 1(b) ). This is perhaps not surprising as others have reported decreased stability of VSV-G with trypsin treatment [50, 51] . Trypsin is a serine protease that cleaves carboxyl groups on the cell surface to remove cells from a culture surface. VSV-G is a cell surface protein that would be exposed to the dissociation reagent [52] . The disruption to VSV-G by trypsin was corroborated by evaluating the number of VSV-G proteins per cell. The administration of trypsin significantly reduced the number of VSV-G proteins on the cell surface of vMSCs (Figure 1(d) ). Thus we replaced trypsin with Accutase, a mixture of proteases and collagenases that has been shown to improve cell viability compared to trypsin [53] . With Accutase treatment, the average number of cells expressing VSV-G after cell harvest was 21% ± 7%, a significant improvement over treatment with trypsin and at a level high enough to discern whether expression of VSV-G can impact MSC-CM fusion (n = 1 replicate per sample per trial for 3 trials). Fusogens such as VSV-G are most potent on the cell surface when there are adequate amounts of protein to facilitate fusion, but a low enough amount to avoid immune responses. Thus VSV-G protein expression per cell was determined using a Quantum MESF kit (Bangs Laboratories, Inc.), on the FACSCalibur. Using this method, the average number of VSV-G proteins per cell was 8 × 10 4 ± 2 × 10 4 with trypsin and 1 × 10 6 ± 8 × 10 3 with Accutase (n = 1 replicate per sample per trial for 3 trials, Figure 1(c) ). Thus, all further experiments were performed using Accutase as the dissociation reagent to prevent VSV-G cleavage.