(A) Immunostaining results of human myoblasts with retroviral expression of NICD alone or together with Dll1. Scale bar, 100 μm. (B) qPCR results of human myoblasts with retroviral expression of NICD alone or together with Dll1. Cells were differentiated for 24 hours. n = 3. (C) Schematic of Notch signaling and cell mixing experiment. (D–F) Myosin immunostaining (D) and qPCR (E, F) results of the co-cultured myoblast-fibroblast cells with expression of full length or mutated Dll1 proteins. EV, empty vector control. Note that deletion of extracellular domain abolishes the cis-inhibitory function of Dll1. (G) Measurement of myoblast fusion as shown in panel D. Treatments in groups 1–5 are same for panels D–G. Data are means ± SEM. *P < 0.05, **P < 0.01, ***P < 0.001. ns, not significant.