AbstractLambda‐producing B lymphocytes have frequently deleted one or, more often, both Igχ loci. This deletion is mediated by the rearrangement of an element which lies 3′ of Cχ and which is called RS (recombining sequence) in the mouse and Kde (kappa‐deleting element) in the human. The tight correlation between Vλ to Jλ rearrangements and an RS‐mediated deletion may indicate that sequences in the Cχ‐RS region are controlling the activation of the Igλ locus. We have linked the Cχ exon and the RS element byphage cloning and compared the Cχ‐RS region to the previously cloned human Cχ‐Kde region. The distance between Cχ and RS is 25 kb and is thus similar to the distance of 24 kb separating the human Cχ exon and Kde element. Both mouse and man carry a conserved sequence of 470 bp (Rx) which lies 9 kb 3′ of the mouse Cχ and 12 kb 3′ of the human Cχ exon. The conserved mouse Rx sequence contains part of the χ3′ enhancer.