For example, carcinoembryonic antigen (CEA) is often highly expressed in a variety of tumors such as gastrointestinal tumors, lung cancer, etc. Detecting the level of CEA in brain metastasis animal models can not only assist in determining whether the tumor has metastasized from the primary foci but also assess the developmental process of the tumor as well as therapeutic efficacy by monitoring the dynamic changes [73]. The gene discussed is CEACAM5; the disease is lung cancer.