Proapoptotic effect of erlotinib on EGFR-negative myeloid cell lines. (A) Absence of EGFR expression on KG-1, P39, and HL60 cells, as determined by immunoblot. A549 cells were included as positive control. (B) Representative annexin V-FITC/PI stainings of erlotinib-treated KG-1 cells, 3 days after treatment, as compared with DMSO-only (0.02%) treated controls. (C-E) Quantitation of dying (annexin V⁺/PI⁻) or dead (annexin V⁺/PI⁺) KG-1 (C), P39 (D), or HL60 (E) cells, 3 or 6 days after culture with the indicated concentrations of erlotinib. (F,G) Influence of erlotinib on the number of viable KG-1 (F) and P39 and HL60 (G) cells. Results are means plus or minus SD of triplicates. These experiments were repeated at least 3 times, yielding comparable results.