Mechanisms that underlie the patterning of cytokine expression in T helper (T_H) cell subsets remain incompletely defined. An evolutionarily conserved ∼400-bp noncoding sequence in the intergenic region between the genes Il4 and Il13, designated conserved noncoding sequence 1 (CNS-1), was deleted in mice. The capacity to develop T_H2 cells was compromised in vitro and in vivo in the absence of CNS-1. Despite the profound effect in T cells, mast cells from CNS-1^−/− mice maintained their capacity to produce interleukin 4. A T cell–specific element critical for the optimal expression of type 2 cytokines may represent the evolution of a regulatory sequence exploited by adaptive immunity.