LIG4 and Down syndrome: A number of genetic disorders associated with severe radiosensitivity have been reported, such as Lig-IV syndrome (DNA ligase IV mutations), A-T-like disorder (ATLD; MRE11 mutations), NBS-like disorder (NBSLD; Rad50 mutations), severe combined immunodeficiency (SCID; DNA-PKcs mutations) and Fanconi anaemia (FANC gene mutations).