Immune repertoires as a distinctive feature of antigen receptor (V(D)J) rearrangements have been explored in several IEIs [22], including RAG deficiency (TCRβ and BCR) [23], Ataxia-telangiectasia (AT) (TCRα and BCR) [24, 25], Cernunnos deficiency (TCRβ, TCRδ, and BCR) [4], Wiskott-Aldrich syndrome (TCRβ) [26], and common variable immunodeficiency (CVID) (TCRβ, BCR) [27, 28]. The gene discussed is BCR; the disease is ataxia telangiectasia.