TCR rearrangement involves sequential deletions of intervening DNA sequences in germline DNA resulting in episomal DNA circles (TREC). Dβ-Jβ excision occurs earlier than sj TREC generation, with thymocyte proliferation occurring between the 2 steps. In panel A, greater intrathymic expansion results in higher thymic T-cell output and a higher ratio of sj/Dβ-Jβ TREC when compared to panel B where less intrathymic expansion results in lower thymic output and a lower ration of sj TREC to Dβ-Jβ TREC. With the same expansion seen in panel A (and the same TREC ratio), decreased thymic output would indicate changes either earlier (at the T-cell progenitor stage) or later (such as apoptosis of postrearrangement thymocytes) than the 2 rearrangement steps.