p27Kip1 is a key cell-cycle regulator whose level is primarily regulated by the ubiquitin-proteasome degradation pathway. Its β1 subunit is one of seven β subunits that form the β-ring of the 20S proteasome, which is responsible for degradation of ubiquitinated proteins. We report here that the β1 subunit is up-regulated in oesophageal cancer tissues and some ovarian cancer cell lines. It promotes cell growth and migration, as well as colony formation. β1 binds and degrades p27Kip1directly. Interestingly, the lack of phosphorylation at Ser158 of the β1 subunit promotes degradation of p27Kip1. We therefore propose that the β1 subunit plays a novel role in tumorigenesis by degrading p27Kip1.