Studying a cell line from a patient with Fanconi anemia, BD0952, where the FANCI protein was expressed at full levels but not ubiquitinated. This cell line had two nonsynonymous mutations in FANCI that were candidates for being causal — P55L and R1285Q. They created constructs with neither, one of each, and both of these variants and functionally characterized them. P55L alone behaved as wildtype, while 1285Q or 55L+1285Q failed to be ubiquinated. This indicated 55L was a nonpathogenic polymorphism and 1285Q was the disease causing mutation.