At least five d_h genes of human immunoglobulin heavy chains are encoded in 9-kilobase dna fragments

The variable region of immunoglobulin (Ig) heavy chain is encoded by three separate genes: variable (V_H), diversity (D_H) and joining (J_H) genes on the germ-line genome. In mice, most complementarity determining region (CDR) 111 of the heavy chains of myelomas and hybridomas sequenced so far can be assigned to one of the 12 already identified germ-line D_H genes by the homology of nucleotide sequences of D_H gene-coding regions although extranucleotides, the so-called N segments, are found at the boundaries between D_H and J_H as well as V_H and D_H. On the other hand, Siebenlist et al. (Nature 1981. 294: 631) identified two D_H gene families in human genome: D_HQ52, located at 45 bp upstream of the J_H gene cluster, and another family encoded at 9-kb regular intervals possibly between V_H and J_H gene clusters. However, the somatic D_H sequences found in V_H-D_H-J_H structure (the somatic D_H segment being defined as the region which is not encoded either by germ-line V_H or J_H gene) are relatively long and apparently random, and do not seem to have the homology to any of the germ-line D_H sequences. To explain the origin of high diversity in the CDR III of human Ig heavy chains, Siebenlist et at. predicted the presence of another mechanism, namely D_H-D_H joinings. In the present study, we identified five D_H genes in one of the above 9-kb repeats. This suggests that the total number of germ-line D_H genes is much higher in man than in mouse. The comparison between somatic DH sequences and germ-line D_H sequences indicates that most somatic D_H sequences in human Ig heavy chains are also produced by VH-D_H and D_H-J_H joinings without the joining of multiple D_H gene segments.