In conclusion, we provide the first evidence that miR-21 expression in malignant CTCL cells is (i) induced by IL-2 and IL-15, key growth factors for malignant T cells, (ii) mediated via STAT5, and (iii) inhibited by a clinical grade JAK3 inhibitor. The gene discussed is STAT5B; the disease is primary cutaneous T-cell non-Hodgkin lymphoma.