A, samples were prepared as described in the legend to Fig. 3B and subjected to immunoblotting for Bcl-2 (top panel) or Bcl-xL (lower panel), as indicated. The phosphorylated forms of Bcl-2 and Bcl-xL and the 150-kDa species of Bcl-xL are indicated by arrows, and molecular mass standards (in kDa) are shown on the right. The samples run in the right two lanes of each panel were treated with β-mercaptoethanol (β-ME) to reverse cross-links. VBL, vinblastine. B, KB-3 cells were untreated or treated with 30 nM vinblastine (VBL), 30 nM vincristine (VCR), 30 nM paclitaxel (TAX), 1 μM doxorubicin (DOX), or 15 μM etoposide (VP-16). The samples were prepared as for Fig. 3B and subjected to immunoblotting for Bcl-xL. Monomeric unphosphorylated and phosphorylated Bcl-xL and the 150-kDa species are indicated.