The differentiation of T helper cells into distinct functional subtypes is critical for mediating effective adaptive immune responses. The molecular mechanism by which a Th precursor cell (Thp) adopts a Th1 or Th2 phenotype involves a complex interplay between the induction of lineagespecific transcription factors, such as T-bet, Gata3, and c-Maf, and the stimulatory effects of the cytokine mil...