XRCC4 and Immunodeficiency: The syndromic characteristics of the newly identified XRCC4-defective patients are globally conform (apart from the embryonic lethality) to the phenotype observed with various XRCC4 conditional/rescued mice except for one major aspect: These patients do not suffer from any sign of immunodeficiency and their immune system develops normally, apart for a mild lymphopenia noted in some cases.