Figure 1.

T-cell differentiation and T-cell receptor excision circle formation. (A) Lymphoid progenitors, which have developed from haematopoietic stem cells in the bone marrow, migrate to the thymus to complete their antigen-independent maturation into functional T cells. In the thymus, the T cells develop their specific T-cell markers and undergo thymic education through positive and negative selection. (B) Because TCRD gene segments are interspersed within TCRA gene segments along chromosome 14q11 (TCRAD locus), following V(D)J recombination of the TCRD locus, the entire locus is preferentially deleted by δREC–ψJα rearrangements. This process gives rise to a δREC–ψJα signal joint on an excision circle (TREC) and a δREC–ψJα coding joint in the genome.