Mutations of the signal transducer and activator of transcription 3 (STAT3) gene on 17q21.2 and deficiency of DOCK8 (dedicator of cytokinesis 8) on 9p24.3 are now known to cause a combined immunodeficiency rendering most of the affected patients susceptible to viral, fungal, and bacterial infections. This evidence concerns the gene STAT3 and severe combined immunodeficiency.