(a) Schematic showing a functional assay to screen for muscle genes that could activate fusion of GFP+ myomaker+ fibroblasts. Representative images of GFP+ cells and nuclei after expression of the indicated genes. Arrows depict cells with multiple nuclei. (b) Illustration of cell mixing approach to show fusion between the populations of fibroblasts. Co-localization of GFP and NLS-TdTomato (NLS-Tom) in the nucleus represents fusion. Representative images demonstrate fusion of myomaker+ myomerger+ fibroblasts but not empty-infected myomaker+ fibroblasts. Arrows indicate fusion between GFP+ and NLS-Tom+ fibroblasts. The percentage of nuclei in syncytia after expression of empty or myomerger (n=3). Data are presented as mean±s.e.m. *P<0.05 compared with empty using an unpaired t-test. Scale bars, 50 μm.