It should be noted that it is difficult to monitor progression beyond one cell cycle in this assay since the iC2C12-FRG1 myoblasts rapidly lose synchronicity. In this study, we demonstrate decreased proliferation rates of myoblasts expressing FRG1, an attribute that could contribute to the long term reduction in muscle regenerative potential and NSC 136476 muscular dystrophy observed in transgenic mice overexpressing FRG1. We have verified expression and muscular dystrophy in the H-FRG1TG mouse and seen that thigh-derived myoblasts, but not diaphragm-derived myoblasts, from these animals demonstrate a proliferative defect by clonal analysis. We also find that induction of FRG1 in myoblasts by a tetracycline-responsive system negatively affects proliferation as determined in cell cycle profiles measured by flow cytometry and hypophosphorylation of pRb. Reduced myoblast proliferation is not commonly linked to muscular dystrophy, which is more classically attributed to death of muscle fibers, such as in Duchenne muscular dystrophy, or a combination of enhanced fiber BAY-60-7550 degeneration and defective differentiation kinetics such as in Lmna2/2 mouse models. However, there have been reports of depressed proliferation kinetics in Duchenne muscular dystrophy myoblasts, compounding the existing mechanisms of muscular dystrophy. Since the proliferation defect gets more severe in myoblasts isolated from HFRG1 TG mice of increasing age, it is difficult to differentiate between two models: that the proliferation defect precedes onset of the dystrophic phenotype or that the defect derives from reduced satellite potential with age resulting from increasing strain on satellite cells to repair damage. Our findings in C2C12 cells may indicate that a cell cycle defect can occur as a primary result of FRG1 overexpression. However, it ultimately remains unclear precisely how FRG1 impacts cell cycle progression. FRG1 has been shown to localize to Cajal bodies in the nucleus, where it is reported to regulate RNA processing. Misprocessing of RNA transcripts has also been linked to myotonic dystrophy. One possible hypothesis for induction of G1 arrest by FRG1 involves altered splicing of transcripts encoding cell cycle components. For instance, altered processing of the cyclin E RNA produced different isoforms of the protein with different affinities for Cdk2. One contentious observation regarding overexpression of FRG1 in patients with FSHD is that other research groups have been unable to replicate the results published showing increased FRG1 transcript in affected muscle. Some studies of myoblasts subjected to microarray analysis or measuring RNA transcription of FRG1 have not yielded any results showing an increase of FRG1 transcript, while similar experiments with qRT-PCR done in other groups have shown an increased trend.