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Faray is a 250 ha island in Orkney, uninhabited by humans since 1946. The only small mammal is the house mouse, Mus domesticus, which between 1982 and 1986 fluctuated in numbers from a maximum of 400–500 to less than 50. Over the period when the population was at its smallest, the frequency of Hbbs increased from 29.1% to 46.6%. There was also a decrease in the frequency of a Robertsonian translocation, Rb (4.10) from 36.4% to 13.3% during the study period; two other Robertsonian chromosomes, Rb (3.14) and Rb (9.12), were always homozygous. The change at the Hbb locus is probably the result of genetic drift; this conclusion was reached only after other possibilities were excluded.