Development of the recently described Th9 cells is selectively and dynamically controlled by epigenetic modifications. The selective epigenetic inactivation of the PU.1 promoter associated with diminished Th9 cell differentiation by naive CD4 T cells allows the assumption of a special physiologic role of IL-9. Once deregulated, IL-9 seems to play an important role in the pathogenesis of several autoimmune disorders.