Notably, a number of previous studies predicted monoubiquitination of FANCD2 to be essential for all of its functions because the non-ubiquitinatable FANCD2 mutant (human FANCD2K561R; Xenopus FANCD2K562R) was not detectable on chromatin and did not rescue the FA phenotype of FANCD2-deficient cells. This evidence concerns the gene FANCD2 and Friedreich ataxia.