XRCC4 and Immunodeficiency: LigIV/XRCC4 is important for normal growth because the knockout of either LIG4 or XRCC4 gene is embryonic lethal [124,145,146], and moreover, hypomorphic mutations of LigIV or XLF in human cause rare diseases characterized conventionally by radiosensitivity, immunodeficiency, microcephaly, etc.; growth retardation and microcephaly caused by mutations in LIG4 are classified as LIG4 syndrome [147,148].