A histological examination of the brain tumor showed that the tumor cells exhibited large-size nuclear mimicking as the primary malignant melanoma metastasis; however, this tumor showed that melanoma markers were completely negative, while tumor cells were positive for thyroid transcription factor 1 (TTF-1), cytokeratin 7 (CK7), AE1/AE3, and epidermal growth factor receptor (EGFR), suggesting a possibility of another malignant tumor, such as lung adenocarcinoma. This evidence concerns the gene TTF1 and cancer.