We used the Sleeping Beauty (SB) transposable element as a tool to probe transposon–host cell interactions in vertebrates. The Miz-1 transcription factor was identified as an interactor of the SB transposase in a yeast two-hybrid screen. Through its association with Miz-1, the SB transposase down-regulates cyclin D1 expression in human cells, as evidenced by differential gene expression analysis using microarray hybridization. Down-regulation of cyclin D1 results in a prolonged G 1 phase of the cell cycle and retarded growth of transposase-expressing cells. G 1 slowdown is associated with a decrease of cyclin D1/cdk4-specific phosphorylation of the retinoblastoma protein. Both cyclin D1 down-regulation and the G 1 slowdown induced by the transposase require Miz-1. A temporary G 1 arrest enhances transposition, suggesting that SB transposition is favored in the G 1 phase of the cell cycle, where the nonhomologous end-joining pathway of DNA repair is preferentially active. Because nonhomologous end-joining is required for efficient SB transposition, the transposase-induced G 1 slowdown is probably a selfish act on the transposon’s part to maximize the chance for a successful transposition event.