First, by comparing the brain metastases and paired adjacent noncancerous brain tissue in 10 patients who suffered from lung cancer, we found that 12 genes were upregulated and 14 genes were downregulated in brain metastases, among which indoleamine 23‐dioxygenase 1 (IDO1), RAR‐related orphan receptor C (RORC), carbonic anhydrase 4 (CA4), and neural cell adhesion molecule 1 (NCAM1) were the top‐ranked upregulated or downregulated genes (Figure 1A,B). Here, CA4 is linked to lung cancer.