This hypothesis has recently been corroborated by studying the functional bias of malignant T cells in L-CTCL patients in whom the malignant T cell clone could be conclusively identified by staining with antibodies against TCR V β. Flow cytometrical assessment of the intracellular cytokine production demonstrated that both the malignant clone and surprisingly the remaining benign T cells expressed high levels of the Th2 cytokines IL-4 and IL-13 and negligible levels of IFN-γ upon activation [33]. This evidence concerns the gene IL13 and primary cutaneous T-cell non-Hodgkin lymphoma.