Genetic and environmental factors shape an individual's susceptibility to autoimmunity. To identify genetic variations regulating effector T cell functions, we used a forward genetics screen of inbred mouse strains and uncovered genomic loci linked to cytokine expression. Among the candidate genes, we characterized a mitochondrial inner membrane protein, TMEM11, as an important determinant of Th1 responses. Loss of TMEM11 selectively impairs Th1 cell functions, reducing autoimmune symptoms in mice. Mechanistically, Tmem11