Building on a decade of in vivo experimental studies using targeted radiotracers, recent proof-of-concept and Phase I trials have deployed a radiotracer comprised of an 89Zr atom tethered to a minibody (engineered antibody) specific for human CD8 protein to evaluate whole-body cytotoxic T-cell distribution and tumor infiltration [47,48,49,50]. The gene discussed is CD8A; the disease is neoplasm.