ATL, CTCL and AITL are thought to arise from activated, TFH-like and/or TREG-like CD4 T cells (50, 94–96), and Card11M365K mutation causes cell-autonomous accumulation of activated, TFH, TREG and TFR CD4 T cells. Here, CD4 is linked to primary cutaneous T-cell non-Hodgkin lymphoma.