FIGURE 23-30 Organization and recombination of TCR loci. The organization of TCR loci is in principle similar to that of immunoglobulin loci (see Figure 23-15). Left: The TCR β-chain locus includes a cluster of V segments, a cluster of D segments, and several J segments, downstream of which are two constant regions. The arrangement of the recombination signals is such that not only is D-J joining allowed, but also V-D-J joining. Direct V-J joining in the TCRβ locus is not observed. Right: The TCR α-chain locus is composed of a cluster of V segments and a large number of J segments. SS = exon encoding signal sequence; Enh = enhancer.