Abstract
Newborn screening for severe combined immunodeficiency (SCID) has developed from the realization that infants affected with SCID require prompt diagnosis and treatment to avoid fatal infectious complications. Screening DNA from infant dried blood spots for T-cell receptor excision circles (TRECs), byproducts of normal antigen-receptor gene rearrangement, has proven to be a reliable method to identify infants with SCID and other serious T lymphocyte defects before the onset of serious infections. The experience of the SCID newborn screening program in California after screening over 3 million infants demonstrates the effectiveness of this measure.