ADA2 and immunodeficiency disease: Bi-allelic loss-of-function mutation in ADA2 located on chromosome 22q11.1 results in loss of enzyme activity of adenosine deaminase 2 (ADA2) [1, 2], a dimeric 57 kDa extracellular isoform of adenosine deaminase 1 (ADA1: deficiency of which causes severe combined immunodeficiency) [1, 9, 10].