In Arrieta et al. [27], there was a significant correlation between high CEA levels and the development of brain metastases, with serum CEA = 40 ng/mL as the boundary (RR = 11.4, 95% CI: 1.7–74, P < 0.01).This study also found that serum CEA levels were significantly higher in the lung adenocarcinoma brain metastasis group than in the non-brain metastasis group. This evidence concerns the gene CEACAM5 and lung adenocarcinoma.