Similarly to our prophylactic arm13, a biopsy obtained at the time of cGVHD showed lower TTCR-C4 frequencies in tissue ( ~ 0.08% of total CD3+ T cells) (Supplementary Fig. 4) compared to PB (60/mcL, ~8% of total CD8+ T cells), suggesting no correlation between PB TTCR-C4 concentration and tissue GVHD. The gene discussed is C4A; the disease is graft versus host disease.