After combining with the “floxed” endogenous Sufufl allele, we found that human SUFU cDNAs (hSUFUs) expressed from the Rosa26 locus are fully capable of supporting embryonic development on their own, but mouse mutants with additional copies of hSUFU genes developed preaxial polydactyly and showed altered cerebellar development and, when crossed into the Ptch1–/+ background, developed MBs with increased penetrance. This evidence concerns the gene SUFU and Mobius syndrome.