Reminiscent to the Xlf-deficient condition, human patients identified with X4 mutations present DNA repair defect hallmarks, including dwarfism, microcephaly, increased cellular sensitivity to radiomimetic agents, but, strikingly, no immunodeficiency (de Villartay, 2015; Saito et al., 2016). The gene discussed is NHEJ1; the disease is Immunodeficiency.