Park JY, et al. - In lung adenocarcinoma patients, researchers conducted this retrospective cohort study to examine the prognostic impact of thyroid transcription factor (TTF)-1 expression according to epidermal growth factor receptor (EGFR)-sensitizing mutation status. Participants in the study were 173 patients with stage IV lung adenocarcinoma. In 84 patients, EGFR-sensitizing mutations have been identified. In stage IV lung adenocarcinoma patients with and without EGFR-sensitizing mutations, TTF-1 expression was a good prognostic indicator for overall survival and progression-free survival. In addition, even in EGFR mutation-positive cases, if the expression of TTF-1 is negative, the EGFR mutation may not be the driving oncogene. In these patients, EGFR-TKI should be used with caution.