AbstractPotent anti-cancer compounds FR901464 and its methyl-ketal derivative spliceostatin A (SSA) inhibit cell cycle progression at G1 and G2/M phases. These compounds bind to the spliceosome and inhibit the splicing reaction. However, the molecular mechanism underlying G1 arrest after SSA treatment remains unknown. In this study, we found that ~90% of SSA-treated cells arrested at G1 phase after cell cycle synchronization. SSA treatment caused upregulation of the p27 cyclin-dependent kinase inhibitor both at mRNA and protein levels. In addition to p27, we observed expression of p27*, a C-terminal truncated form of p27 that is translated from CDKN1B (p27) pre-mRNA accumulated after splicing inhibition. Overexpression of p27 or p27* inhibited the exit from G1 phase after a double thymidine block. Conversely, knocking down of p27 by siRNA partially suppressed the G1 phase arrest caused by SSA treatment. There results suggest that G1 arrest in SSA-treated cells is caused, at least in part, by upregulation of p27 and p27*.