5% gelatin coated chamber slides in RPMI-B27 medium supplemented with 10% FBS, 10 uM Y-27632 (Sigma), Polybrene (Millipore) and 5000 particles per cell of a lentivirus expressing Cre under the striated muscle-specific CK7 regulatory cassette, based on the muscle creatine kinase promoter (MCK) [9–11].