Lymphocyte development is controlled by dynamic repression and activation of gene expression. These developmental programs include the ordered, tissue-specific assembly of Ag receptor genes by V (D) J recombination. Changes in gene expression and the targeting of V (D) J recombination are largely controlled by patterns of epigenetic modifications imprinted on histones and DNA, which alter chromatin accessibility to nuclear factors. An important component of this epigenetic code is methylation of histone H3 at lysine 9 (H3K9me), which is catalyzed by histone methyltransferases and generally leads to gene repression. However, the function and genetic targets of H3K9 methyltransferases during lymphocyte development remain unknown. To elucidate the in vivo function of H3K9me, we generated mice lacking G9a, a major H3K9 histone methyltransferase, in lymphocytes. Surprisingly, lymphocyte development is unperturbed in G9a-deficient mice despite a significant loss of H3K9me2 in precursor B cells. G9a deficiency is manifest as modest defects in the proliferative capacity of mature B cells and their differentiation into plasma cells following stimulation with LPS and IL-4. Precursor lymphocytes from the mutant mice retain tissue-and stage-specific control over V (D) J recombination. However, G9a deficiency results in reduced usage of Igλ L chains and a corresponding inhibition of Igλ gene assembly in bone marrow precursors. These findings indicate that the H3K9me2 epigenetic mark affects a highly restricted set of processes during lymphocyte development and activation.