Introduction: Insulin receptor substrate 1 (IRS1) is one of the major mediators of IGF signals . Recently, we have reported that L6 myoblasts stably expressing IRS1 (L6-IRS1) were not able to differentiate into myotubes . We have also shown that constitutive Foxo1 nuclear exclusion is one of the reasons why this stable line could not differentiate . The present study was undertaken to elucidate alternative molecular mechanisms underlying the inhibition of myogenesis by constitutive IRS1 expression . Method and results: At first, cell numbers were counted during proliferation in L6-GFP control cells or L6-IRS1 cells . Before confluence, we did not detect any differences between these two cell lines . However, even after reaching confluence L6-GFP control cells could continue to proliferate and cell number increased, whereas L6-IRS1 cells did not proliferate further and cell number decreased instead . In addition, after induction of differentiation, cell density gradually increased in control cells, whereas cell density decreased in L6-IRS1 cells . In order to test the migrating activity, L6-GFP cells or L6-IRS1 cells were overlaid on L6-mock cell monolayer . L6-GFP cells migrated into the confluent cell monolayer, but L6-IRS1 cells did not . Next, the mixture of L6-GFP cells and L6-IRS1 cells at a ratio of 1:9 were cultured for 8 days . Two days later, only 20% of cells were control cells, but control cells occupied more than 70% of cell monolayer 8 days later . In addition, immunofluorescent analyses revealed that L6-IRS1 cells surrounded by control cells showed condensed nuclei and cleaved Caspase3 expression, indicating that L6-IRS1 cells underwent apoptosis . These data indicated that constitutive expression of IRS-1 causes cell competition . Conclusion: Taken together we hypothesized that during differentiation, cells compete with each other and that cells highly expressing IRS-1 undergo apoptosis . IRS-1 constitutive expression causes this cellular competition, leading to defective in myogenic differentiation .