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using a yeast two-hybrid screen and in vitro binding assays. Our studies demonstrate that Kbtbd5 interacts and regulates the cytoplasmic localization of DP1. GST pulldown assays demonstrate that the dimerization domain of DP1 interacts with all three of the Kbtbd5 domains. We further show that Kbtbd5 promotes the ubiquitination and degradation of DP1, thereby inhibiting E2F1-DP1 activity. To investigate the in vivo function of Kbtbd5, we used gene disruption technology and engineered Kbtbd5 null mice. Targeted deletion of Kbtbd5 resulted in postnatal lethality. Histological studies reveal that the Kbtbd5 null mice have smaller muscle fibers, a disorganized sarcomeric structure, increased extracellular matrix, and decreased numbers of mitochondria compared with wild-type controls. RNA sequencing and quantitative PCR analyses demonstrate the up-regulation of E2F1 target apoptotic genes (Bnip3 and p53inp1) in Kbtbd5 null skeletal muscle. Consistent with these observations, the cellular apoptosis in Kbtbd5 null mice was increased. Breeding of Kbtbd5 null mouse into the E2F1 null background rescues the lethal phenotype of the Kbtbd5 null mice but not the growth defect. The expression of Bnip3 and p53inp1 in Kbtbd5 mutant skeletal muscle are also restored to control levels in the E2F1 null background. In summary, our studies demonstrate that Kbtbd5 regulates skeletal muscle myogenesis through the regulation of E2F1-DP1 activity.