Laboratory work-up revealed normal levels of adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP), and absence of mutations in genes associated with immunodeficiency including RAG1, RAG2, CD3D, CD3E, and DCLRE1C. Although lymphocyte defects and impaired growth can be caused by inherited defects in DNA repair genes, DNA sequencing did not reveal evidence of DNA ligase IV deficiency, Cernunnos defects, ataxia telangiectasia, Nijmegen breakage syndrome, Bloom syndrome, or Fanconi anemia. Here, CD3D is linked to ataxia telangiectasia.