Cernunnos-XLF deficiency (MONDO_0012650) is a rare form of combined immunodeficiency characterized by microcephaly, growth retardation, and T and B cell lymphopenia. Also known as: Cernunnos XLFD, Cernunnos deficiency, NHEJ1 deficiency, combined immunodeficiency-microcephaly-growth retardation-sensitivity to ionising radiation syndrome, combined immunodeficiency-microcephaly-growth retardation-sensitivity to ionizing radiation syndrome.