T-cell receptor excision circles (TRECs) are small circles of DNA created in T-cells during their passage through the thymus as they rearrange their TCR genes. Their presence indicates maturation of T cells; TRECs are reduced in SCID (Severe combined immunodeficiency disease).
Single joint T-cell receptor excision circles (sjTRECs) were in the news as a way to test the age of the individual from a blood sample. The detection of sjTRECs can be further used as a diagnostic tool to monitor the thymic output e.g. following hematopoietic stem cell transplantation or in case of AIDS