To determine if vMSCs fuse more readily with cardiomyocytes than untreated MSCs, vMSCs and MSC controls were cocultured with CMs and analyzed for incidence of fusion. To distinguish cell types in cocultures, MSCs and CMs were stained with 1 μm Cell-Tracker Green CMFDA and 20 μm Red CMTPX (Molecular Probes Eugene, OR, USA), respectively, according to the manufacturer's protocol. Following labeling, 5 × 10 5 CMs were plated and cultured for 4 h followed by the addition of 1.5 × 10 5 MSCs or vMSCs per well in 6-well plates (BD Biosciences). After 14 h of coculture, suspensions were washed with 1X PBS and then bathed for 2 min in fusion media [46] of varying pH (i.e., pH 5.5, 6.5, or 7.5 that correspond to active and inactive forms of the VSV-G fusion protein) adjusted with HCl. For long-term characterization of fusion products, medium was changed 1 day and 4 days after coculture.