TREC and KREC copy numbers in dried blood spot samples (DBSS) from anonymized Guthrie cards and retested samples and in patients diagnosed with SCID, XLA, AT, NBS, X-HIGM, CVID, or IgAD. Dot size correlates with the amount of ACTB per sample. Dashed lines represent cutoff values for TRECs/μL and KRECs/μL, respectively. Proven molecular defects in the shown SCID patients are depicted as follows: RAG1 indicates recombination activating gene 1; IL2RG, interleukin 2 receptor γ chain (X-SCID); AK2, adenylate kinase 2; IL7RA, interleukin 7 receptor subunit α; JAK3, Janus kinase 3; and uncl., unclassified defect.