Severe combined immunodeficiency (SCID) is a group of life-threatening diseases. SCID is caused by mutations that prevent the normal function of the immune system. Infants born with SCID are at very high risk of infectious diseases. One feature of SCID is that T-lymphocytes do not develop normally. In the development of normal T-lymphocytes, the production of circular pieces of DNA called T-lymphocyte receptor excision circles (TRECs) is an important event. It is possible to use the polymerase chain reaction (PCR) to detect TRECs in DNA extracted from a sample of blood. The results of this reaction can be used to identify children with SCID.