Correspondingly, DNA-PKcs protein expression is preserved in the two patients with DNA-PKcs deficiency identified thus far—one patient carries the L3062R mutation in the FAT domain, with preserved kinase activity and isolated SCID [79], and the other one has reduced kinase activity with SCID and severe microcephaly [80], similar to patients with hypomorphic mutations in LIG4 or XRCC4 [81–83]. The gene discussed is XRCC4; the disease is severe combined immunodeficiency.