High activity levels of STAT5A and STAT5B variants in the hematopoietic system of transgenic mice can lead to a lethal condition resembling human peripheral T-cell lymphoma (PTCL) and elevated expression of STAT5A/B has been detected in human PTCL samples. The gene discussed is STAT5B; the disease is mature T-cell and NK-cell non-Hodgkin lymphoma.