Non-homologous end-joining factor 1, also known as NHEJ1, belongs to the XLF family. NHEJ1 was originally discovered as the protein mutated in five patients with growth retardation, microcephaly, and immunodeficiency. The protein is required for the non-homologous end joining (NHEJ) pathway of DNA repair. Patients with NHEJ1 mutations also have immunodeficiency due to a defect in V(D)J recombination, which utilizes NHEJ to promote immune system diversity.