Figure 1:

Pictorial explanation of T-cell receptor excision circle (TREC)–based newborn screen for severe combined immunodeficiency (SCID). The healthy infant (A) has an intact immune system; the thymus produces a normal repertoire of T cells. As unique T cells are produced, TRECs are released into the systemic circulation and can be quantified on the newborn blood spot. Despite appearing healthy externally, the infant with SCID (B) has very low or absent T-cell production. This leads to undetectable numbers of TRECs on the newborn screen, facilitating early diagnosis of SCID.