Here we utilize viral machinery from vesicular stomatitis virus (the glycoprotein, VSV-G), of the Rhabdoviridae family, to induce heterotypic fusion between human MSCs and mouse CMs in vitro and in an in vivo mouse model of myocardial infarction. Following MSC-CM fusion, we tracked the phenotype and morphology of fusion products for one week in vitro and 3 weeks in vivo. VSV-G was selected because it does not require proteolytic cleavage, is the sole mediator of receptor binding and fusion, and is pH dependent [38, 39] . In particular, VSV-G does not require facilitating proteins to either dock to the host membrane prior to fusion, or enzymes to prompt the activation of the fusogen. Furthermore, the pH dependence of VSV-G is advantageous as the local heart pH after acute ischemic injury [40] [41] [42] is within the acidic range needed to initiate a conformational change in VSV-G [38, 39] . In this way, selective activation of VSV-G on transfected MSCs (vMSCs) at the site of myocardial injury should induce local fusion in situ, thereby increasing donor cell engraftment and integration within the tissue and potentially facilitate cardiac differentiation.