Objective:To explore the mechanism of a novel BMX splicing variant induced epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI) gefitinib resistance in lung cancer.Methods:Stable transgenic cell line PC9-BMXΔN and HCC827-BMXΔN were constructed by lentivirus infection of PC9 and HCC827 cells carrying EGFR mutation. The cells were divided into PC9-Vec group (PC9 cells transfected with empty vector), PC9-BMX group (PC9 cells stably expressing BMX), PC9-BMXΔN group (PC9 cells stably expressing BMXΔN) and HCC827-Vec group (HCC827 cells transfected with empty vector), HCC827-BMXΔN group (HCC827 cells stably expressing BMXΔN). Quantitative real-time PCR was used to detect the expression levels of mRNA. The protein expression levels in each group were detected by Western blotting. The cells in the PC9-Vec group and PC9-BMXΔN group were treated with 0, 0.01, 2.00, 50.00, 100.00, 200.00 nmol/L and 2.00, 4.00 μmol/L gefitinib. The cells in the HCC827-Vec group and HCC827-BMXΔN group were treated with 0, 0.01, 1.00, 10.00, 100.00 nmol/L and 1.00 μmol/L gefitinib. MTT method was used to detect cell viabilities.Results:The PC9-BMXΔN cells were scattered and showed a fibroblast-like morphology. Compared with the PC9-Vec cells, the relative expression levels of fibronectin, N-cadherin, vimentin, Snail, Slug and TWIST 2 mRNA in PC9-BMXΔN cells were up-regulated. Compared with the PC9-Vec cells and PC9-BMX cells, the expression levels of fibronectin and vimentin protein in PC9-BMXΔN cells were up-regulated; while the expression level of E-cadherin protein in PC9-BMXΔN cells was significantly down-regulated. Compared with the PC9-Vec cells, the cell viabilities of PC9-BMXΔN cells treated with 0.01 nmol/L [(99.11±2.16)% vs. (91.29±1.91)%, t=-4.701, P=0.011], 2.00 nmol/L [(80.41±1.48)% vs. (63.36±2.14)%, t=-11.324, P<0.001], 50.00 nmol/L [(80.83±5.38)% vs. (60.22±3.61)%, t=-5.507, P=0.005], 100.00 nmol/L [(75.54±3.46)% vs. (59.93±1.91)%, t=-6.836, P=0.002], 200.00 nmol/L [(77.57±6.53)% vs. (56.70±2.88)%, t=-5.064, P=0.007], 2.00 μmol/L [(70.22±3.45)% vs. (53.14±0.89)%, t=-8.309, P=0.001], 4.00 μmol/L [(68.66±4.67)% vs. (52.30±2.59)%, t=-4.882, P=0.008] gefitinib were significantly increased, with statistically significant differences. Similarly, compared with the HCC827-Vec cells, the cell viabilities of HCC827-BMXΔN cells treated with 1.00 nmol/L [(64.36±2.49 )% vs. (47.13±4.21)%, t=-7.067, P=0.019], 10.00 nmol/L [(63.25±5.87)% vs. (43.28±2.95)%, t=-5.267, P=0.006], 100.00 nmol/L [(49.47±5.74)% vs. (37.12±4.92)%, t=-2.830, P=0.047], 1.00 μmol/L [(49.05±3.34)% vs. (32.06±4.73)%, t=-5.073, P=0.007] gefitinib were significantly increased, with statistically significant differences. Gefitinib treatment could significantly inhibit the expression levels of p-EGFR and p-ERK1/2 both in PC9-Vec cells, PC9-BMX cells and PC9-BMXΔN cells. Compared with the PC9-Vec cells and PC9-BMX cells, the expression level of p-EGFR in PC9-BMXΔN cells was significantly increased after gefitinib treatment for 8 h (0.91±0.04 vs. 0.81±0.04 vs. 0.80±0.05, all P<0.05); the expression levels of p-ERK1/2 in PC9-BMXΔN cells were significantly increased after gefitinib treatment for 2 h (0.64±0.06 vs. 0.38±0.12 vs. 0.37±0.14), 4 h (1.28±0.06 vs. 1.08±0.06 vs. 1.11±0.07), and 8 h (0.75±0.04 vs. 0.55±0.05 vs. 0.60±0.07), with statistically significant differences (all P<0.05). Conclusion:BMXΔN is involved in EGFR-TKI gefitinib resistance in lung cancer, which may be achieved by inducing cells to undergo epithelial-mesenchymal transition and activating the ERK/MAPK signaling pathway.