XRCC5 and Severe short stature: Furthermore, Ku80−/− dwarfism is not exclusively postnatal (animals are already smaller at birth [41]), and we found no evidence of perturbation of the GH/IGF-1 postnatal growth axis at 2 wk, as in progeroid NER disorder, or at 10 wk, near the peak of somatotroph axis activity.