ATM and lymphopenia: These findings support the notion that the genomic instability associated with V(D)J recombination at the TCRδ locus is the molecular origin of both lymphocytopenia and the signature t(12;14) translocations associated with ATM deficiency.91 In addition to these findings, it was shown that ATM‐deficient lymphocytes also contain telomere‐deleted ends produced by failed end joining during V(D)J recombination.92