PRA: Type B identifies a risk variant for the disease. (i.e. it is not a definative yes/no risk answer, but may aid in identifying at-risk or low-risk dogs). The Type B PRA test results are reported as causal or "risk variant, because, although dogs carrying two copies of the variant are very likely to develop PRA, a percentage of the research sample set that were homozygous for the variant did not develop PRA by 7 years of age. It is not clear at this time whether the variant represents a causal mutation ?and that other factors influence the ?penetrance? of the mutation.