Using an established B6→Balb/c acute GVHD model, we transplanted recipients with β actin–driven GFP-labeled T cells and observed that CD4+ and CD8+ donor T cells were present in significantly higher frequency and absolute numbers in GVHD as opposed to BM control mice in the prefrontal cortex (PFC), amygdala, brainstem, and cerebellum (Figure 1A) which represent regions of the brain that are critical for the processing of higher cortical function, emotional inputs, regulation of key involuntary actions, and balance, respectively (28–31). The gene discussed is CD4; the disease is graft versus host disease.