Fanconi anemia (FA) is a rare bone marrow failure disorder with defective DNA interstrand crosslink repair. Still, there are FA patients without mutations in any of the 15 genes individually underlying the disease. A candidate protein for those patients, FA nuclease 1 (FAN1), whose gene is located at chromosome 15q13.3, is recruited to stalled replication forks by binding to monoubiquitinated FANCD2 and is required for interstrand crosslink repair, suggesting that mutation of FAN1 may cause FA. Here we studied clinical, cellular, and genetic features in 4 patients carrying a homozygous 15q13.3 micro-deletion, including FAN1 and 6 additional genes. Biallelic deletion of the entire FAN1 gene was confirmed by failure of 3'- and 5'-PCR amplification. Western blot analysis failed to show FAN1 protein in the patients' cell lines. Chromosome fragility was normal in all 4 FAN1-deficient patients, although their cells showed mild sensitivity to mitomycin C in terms of cell survival and G(2) phase arrest, dissimilar in degree to FA cells. Clinically, there were no symptoms pointing the way to FA. Our results suggest that FAN1 has a minor role in interstrand crosslink repair compared with true FA genes and exclude FAN1 as a novel FA gene.