We isolated 10 myoblast clones from Syrian/golden hamster embryo (SHE) cells irradiated with ultraviolet light. They were originally isolated as anchorage-independent clones. All clones showed characteristic morphology of myoblast in culture and formed swirled myofiber colonies specific to myoblast macrocolonies. However, in immunostaining experiments using anti-fast myosin antibody, we found that 4 of 10 myoblast clones were myosin negative (Myo-), while six were myosin positive (Myo+). Western blot analysis confirmed the disappearance of the 200-kDa myosin-specific band in the 4 Myo- clones. Furthermore, these 4 Myo- clones lost the ability to form multinucleated myotubes. Karyotype analysis revealed that all Myo- clones had trisomy of chromosome 7, while Myo+ clones showed no apparent karyotypic change from normal SHE cells. Compared with that of SHE cells, the transcriptional level of the myc gene in Myo+ clones was augmented, but there was no increase of myc gene expression in Myo- clones. Furthermore, the introduction of activated myc gene partially converted the Myo- phenotype to Myo+. These results suggest that trisomy of chromosome 7 and a deficiency in enhanced expression of the myc oncogene are associated with the suppression of both the production of myosin and the formation of multinucleated cells.