 Study Objective: T cell receptor excision circles TRECs are episomal DNA circles excised from the T cell receptor genes during T cell maturation in the thymus. They do not replicate, so they are diluted out as T cells proliferate. Our objectives are to examine the levels of TRECs in blood samples from patients with primary immunodeficiency PI diseases; to determine which PI diseases might be detected by testing newborn dried blood spots for low numbers of TRECs; and to use the blood spots collected for future confirmatory or alternative tests to develop newborn screening for PIs. 