ADA and combined immunodeficiency: Some years later, KREC (kappa–deleting recombination circles) was proposed for a combined TREC—KREC screening approach for severe forms of T- and/or B-cells deficiencies, such as SCID, late-onset adenosine deaminase deficient SCID (ADA-SCID), combined immunodeficiency (CID), or different forms of agammaglobulinemia (XLA) (6, 7).