We have previously shown that an MS-based approach for the quantification of signature peptides for BTK, WASP, and a T-Cell marker CD3ε from tryptic digests of PBMCs can be used to screen X-linked agammaglobulinemia (XLA), Wiskott-Aldrich Syndrome (WAS), and SCID, respectively (26). The gene discussed is CD3E; the disease is Bruton-type agammaglobulinemia.