More recently, efforts have turned to the development of novel imaging agents designed to specifically measure immune cell presence and/or activity within the tumor, including tracers targeting CD3, CD4, CD8, IL-12, IL-2R, granzyme B, inducible co-stimulator (ICOS), and the immunoPET tracer we have developed against interferon gamma (IFNγ) (20–26). Here, CD4 is linked to neoplasm.