tokens: Janus tyrosine kinase ( Jak ) 3 and transcription factors Stat5a and Stat5b are essential for the proliferation of normal T cells and are constitutively hyperactivated in both HTLV-1-transformed human T cell lines and lymphocytes isolated from HTLV-1-infected patients ; therefore , a critical role for the Jak3-Stat5 pathway in the progression of this disease has been postulated .