The Rop (radial opacity) mutation, which was recovered in a mutagenicity screen after paternal treatment with procarbazine, was analyzed to determine phenotype, chromosomal localization, candidate genes, and molecular lesion.Native lenses were photographed under a dissecting microscope. Histologic sections of the eye were made according to standard procedures. Fine mapping of the mutation in relation to microsatellite markers for mouse chromosome 1 was performed. Candidate genes were amplified by PCR from cDNA or genomic DNA and sequenced.The nuclear opacity of the heterozygous mutants showed radial structures, whereas the opacity of the homozygotes was homogenous. The histologic analysis revealed changes in the lens nucleus, which corresponds to the pronounced opacification in lenses of homozygous mutants. The allelism of Rop to the Cat2 group of dominant cataracts on mouse chromosome 1 was confirmed by linkage to microsatellite markers D1Mit156 and D1Mit181. The cluster of the Cryg genes and the closely linked Cryba2 gene were tested as candidates. A T-->A exchange in exon 2 of the Crygf gene leads to a Val-->Glu exchange in codon 38 and was considered to be causative for the cataract phenotype; therefore, Crygf(Rop) has been suggested as the designation for the mutation.Crygf(Rop) is the first mutation affecting the Crygf gene. Dominant cataract mutations for all six Cryg genes on mouse chromosome 1 have now been characterized, demonstrating the importance of this gene cluster in lens transparency.