IL-27 limits IFN- γ production by CD4+ T cells during various infections [3]–[7], attenuates the development, but not necessarily maintenance, of Th17 responses by limiting retinoid-related orphan receptor (ROR)c expression [8]–[11] and stimulates IL-10 production by multiple effector CD4+ T cell populations [12]–[14]. The gene discussed is CD4; the disease is infection.