The translocator protein (TSPO), an 18 kDa mitochondrial membrane protein located on the outer mitochondrial membrane, has become an important biomarker for imaging neuroinflammation, cancer, and other diseases involving immune cell activation.1,2 This makes it a valuable target for positron emission tomography (PET), a non-invasive imaging modality able to quantify molecular processes in vivo. The gene discussed is TSPO; the disease is cancer.