Formation of TRECs. During their passage through the thymus, T-cell precursors rearrange their T-cell receptor (TCR) genes. This step requires the excision of segments of DNA, the ends of which are subsequently ligated to form small circles of episomal DNA (signal and coding), hereafter referred to as T-cell receptor excision circles (TRECs). The sequences remaining after recombination provides the target for a PCR detection of TREC. Each mature T-cell exiting the thymus and released into the circulation, therefore contains episomal TRECs. As these cells replicate in the peripheral blood, TRECs are not replicated but are distributed among daughter cells and are gradually diluted in the peripheral T-cell pool. Therefore, by measuring the proportion of peripheral T-cells containing TRECs, an estimate of recent thymic function can be obtained.