In particular, epidermal keratinocytes that become activated by malignant T cells in vitro and in vivo, have been shown to express IL-15 in situ in CTCL lesions, indicating that malignant T cells may also propagate their own proliferation through stimulation of keratinocytes to produce tumor growth factors such as IL-15 [114, 116]. This evidence concerns the gene IL15 and primary cutaneous T-cell non-Hodgkin lymphoma.