Using metastatic models of EGFR-mutant lung cancer, we show that cancer cells expressing high intracellular S100A9 escape osimertinib and initiate brain relapses...we performed S100A9 immunostaining on tissue specimens that were obtained prior to osimertinib treatment from 29 EGFR-mutant lung cancer patients (Fig. 4a and Supplementary Table 6)...high expression of S100A9 in cancer cells from pre-osimertinib treatment samples correlated significantly with worse PFS on osimertinib (n=29, p=0.0011) both in the combined cohort (Fig. 4d) and when stratified by treatment lines (n = 17, p = 0.0106 for first-line osimertinib patients, and n = 12, p = 0.0451 for second- and third-line patients, Fig. 4e). Therefore, based on our preclinical studies and clinical validation, elevated S100A9 expression in cancer cells is significantly associated with brain metastasis and strongly correlates with progression in osimertinib-treated lung cancer patients.