Enhancers are essential for long range chromatin opening and the activation of V(D)J recombination at the antigen receptor loci. The murine immunoglobulin lambda light chain locus is a duplicated locus and, using a bacterial artificial chromosome spanning the 3 ' half of the locus to generate transgenic mice, we have identified a critical enhancer element for lambda locus recombination. Four hypersensitive sites had been previously mapped downstream of the JC lambda(1) gene segment (HS1-4). Systematic deletion of these individual hypersensitive sites showed that HS1, which forms the major part of the transcription enhancer, E lambda(3-1), is essential for Ig lambda, recombination and that it also helps to restrict Ig lambda stage-specific recombination. (C) 2013 Elsevier Inc. All rights reserved.