Moynahan et al [24] have used such a GFP recombination substrate to demonstrate that cells with defective BRCA2 protein are deficient in their ability to repair the I-SceI-induced DSB through homologous recombination. Expression of I-SceI resulted in 1 out of 1400 cells producing GFP via homologous recombination in the human pancreatic tumor cell line CAPAN-1. CAPAN-1 cells carry a deletion of BRCA2 on one homolog and codes for a protein truncated at amino acid 1981 on the other homolog. The authors indicate that the level of I-SceI-induced recombination in CAPAN-1 is over 100-fold less than that seen using other (BRCA2+) human tumor cell lines. The different lines examined, however, are very likely to differ genetically from CAPAN-1 cells not only at the BRCA2 locus, but also at a very large number of additional loci. This raises the possibility that some or even all of the recombinational repair defect seen in CAPAN-1 could be due to mutations at non-BRCA2 loci.