Human T-Cell Receptor Repertoire Analyzer

In antibody heavy chains (IGH), the D-segment is flanked on both sides by 12-bp spacer RSSs (12-D-12), allowing RAG1/2 recombinase to pair in forward or reverse orientations and producing ~10% inverted D-genes in BCR repertoires. In stark contrast, Human TCR D-segments (TRBD & TRDD) are asymmetrically flanked by a 5' 12-RSS and a 3' 23-RSS (12-D-23). Inversion would force illegal 23-to-23 or 12-to-12 pairings, which are biochemically forbidden. Consequently, TCR D-inversion is virtually non-existent (<0.1%); instead, TCR achieves CDR3 hypervariability through tandem forward concatenation (e.g. TRDD1–TRDD2–TRDD3 in TRD), flexible forward reading frames, and hyperactive TdT N-addition.