Expression levels of the β-type proteasome subunits in proliferating and senescent WI-38 cells. Cytosolic extracts were prepared from proliferating and senescent WI-38 cells. Samples were analyzed by immunoblot using antibodies specific for each of the β-type proteasome subunits. Actin served as a loading control. Levels of p21WAF1 (p21) were also determined to serve as an inducible marker for replicative senescence. Results from duplicate cultures are shown.