Figure 1

T cell Receptor Excision Circles (TRECs). TRECs are episomal DNA circles produced in thymocytes by excisional rearrangements of T cell receptor (TCR) genes; they are stable, not duplicated during mitosis, diluted out with each cell division, and therefore higher in thymocytes, recent thymic emigrants (RTEs) and naïve T cells. Quantitative polymerase chain reaction (PCR) of coding-joint (cj) δRec ψJα TREC, produced at TCRα/δ locus within chromosome 14 (14q11) by > 70% of developing human α:β T cells, counts in the peripheral blood naïve α:β T lymphocytes recently dismetted by thymus: in newborn, values < 25 TRECs/μL indicate SCID.