EGFR and lung adenocarcinoma: The mRNA levels of the MMP-1 gene in two EGFR-TKI–resistant lung adenocarcinoma cells (PC9/GR and PC9/ER) were significantly higher than those in EGFR-TKI–sensitive cells (PC9/6m) (PC9/GR vs. PC9/6m, p = 0.0349; PC9/ER vs. PC9/6m, p < 0.0001; PC9/ER vs. PC9/GR, p < 0.0001) (Figure 1a).