Parthenolide inhibits cell growth (A, B) and induces apoptosis in a concentration-dependent (C) and a time-dependent manner (D). The indicated cell lines were seeded in 96-well plates and treated with the given concentration of PTL for 48 hrs. Cell survival was estimated using SRB assay (A) and MTT assay (B). Points: mean of four replicate determinations; bars: S.D. The indicated cells were treated with indicated concentrations of PTL for 24 hrs (C) or treated with 20 μmol/L PTL for various lengths of time and harvested for Western blot analysis (D). CF: cleaved form.