Erlotinib-induced differentiation of EGFR-negative myeloid cell lines. (A) Representative Wright-Giemsa staining of KG-1 cells cultured 6 days in the absence (control) or presence of DMSO (0.02%), erlotinib (10 μM), or all-trans retinoic acid (ATRA, 1 μM), revealing signs of differentiation, ie, reduced cytoplasmic basophilia and nuclear lobulation, or apoptosis, ie, chromatin condensation (pyknosis) and nuclear fragmentation (karyorrhexis). (B) Quantitation of results, as obtained in panel A, on 100 or more cells. (C-H) Erlotinib-driven differentiation in P39 (C-E) or HL60 (F-H) cells. Differentiation was quantified (X ± SD of triplicates) by immunofluorocytometric measurement of surface CD11b on day 3 (quantified in panels D,G) and day 6 (panels C,D and F,G) or by Wright-Giemsa staining of cytospins on day 6 of the incubation period (panels C,E and F,H). Results are representative of at least 3 independent experiments.