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Objective: To characterize hereditary retinal degeneration in the Abyssinian cat (rdAc) in a recently established closed colony segregating for the rdAc mutation, and evaluate possible differences in the age of onset and progression of disease phenotype since the initial description of rdAc 25 years ago. The sample size of an earlier study was increased in order to determine the allele frequency in Abyssinian and Somali cats on a worldwide basis., Animals Studied: Twenty rdAc affected cats from the closed animal facility, 87 Abyssinian and Somali cats for study of genotype-phenotype concordance, and DNA from 131 Abyssinian and Somali cats from Scandinavia, the UK and Australia for evaluation of the rdAc allele frequency., Procedures: DNA was extracted from blood and buccal swabs using commercially available kits, followed by genotyping. Ophthalmic examinations were performed in the USA and Sweden by two board-certified veterinary ophthalmologists., Results: A greater variation in the age of onset and progression of the disease was observed compared to that previously described. An excellent correlation between genotype and phenotype was observed. A population genetic survey revealed that the rdAc allele is in moderate abundance in the Abyssinian breed in Europe and Australia. Surprisingly, homozygosity for the mutant allele was observed in a Siamese cat with ophthalmoscopic findings similar to those originally described for affected rdAc individuals., Conclusions: Alertness to the potential of rdAc is needed on the part of the veterinary ophthalmology community, not only in Abyssinian and Somali cats but possibly also in other related cat breeds.