Anti–CD20-mIFNα potently induces apoptosis of 38C13-huCD20 cells in a dose-dependent manner. 38C13-huCD20 cells were incubated with various concentrations of the indicated proteins for 48 hours. Staining with annexinV-FITC and PI was performed to distinguish necrotic (annexin⁻PI⁺), early apoptotic (annexin⁺PI⁻), and late apoptotic (annexin⁺PI⁺) cell populations. The percentage of total apoptotic cells was quantified for each sample as the sum of early apoptotic and late apoptotic cells (bottom panel). Experiments were performed in triplicate, and error bars indicate mean ± SD. *P = .001. **P < .001.