As hypomorphic mutations of MRE11 complex members underlie several severe chromosomal instability disorders, this suggests that EXO1 may play a key role in suppressing the severity of the pathological outcomes associated with NBS and ATLD mutations and could genetically interact with less severe alleles of MRE11 complex members in multigenic hereditary disorders. The gene discussed is EXO1; the disease is Nijmegen breakage syndrome.