We then successfully incorporated the SMN1 exon 7 test reagents into the existing multiplex real-time PCR assay for T-cell receptor excision circle (TREC), a marker of T cell production, to detect severe combined immunodeficiency (SCID), which also included the target RPP30 as an internal control for both assays (manuscript in preparation). Here, SMN1 is linked to severe combined immunodeficiency.