A putative diversity segment of immunoglobulin heavy-chain genes (D segments) has been identified 700 base pairs 5' to JH1 DNA on the germ-line genome of the mouse. This 10-base pair D segment is flanked by two sets of sequences related to (SEE FORMULAR IN TEXT) which are possible recognition sites for a recombinase. The spacer separating the heptamer and the nonamer is 12 base pairs long on both sides of the D segment. As the space separating the two signal sequences in VH DNAs and JH DNAs is 23 +/- 1 base pairs long, the two recombinations required for creation of a complete immunoglobulin VH gene, a VH--D joining and a D--JH joining, follow a 12/23-base pair spacer rule. Allelic exclusion is discussed with respect to D segments.