NBN (which encodes for the protein NBS1) hypomorphic mutations lead to Nijmegen breakage syndrome (NBS), which manifests with facial malformations, microcephaly and symptoms similar to AT including radiosensitivity, immune deficiency and increased risk of cancer.59 In murine models, loss of NBS1 hinders T‐cell development at an early developmental stage.60 Mechanistically, NBS1 depletion compromises loading of the MRN complex to V(D)J‐generated DSBs and thereby affects DNA end resection.60 Here, NBN is linked to microcephaly.