HGS-ETR1 mAb– and HGS-ETR2 mAb–induced cell death are always associated with Mcl-1 cleavage. Myeloma cells (3 million/6 mL) were cultured for 7 hours with (NAN-1, NAN-4) or without (KMS12PE, JJN3) 3 ng/mL IL-6 in the presence or absence of 0.6 μg/mL HGS-ETR1 or HGS-ETR2 mAb, as indicated.