Two studies have used linkage analysis to estimate the age of the CCR5 Δ32 deletion, assuming that the amount of recombination and mutation observed on genomic regions surrounding the CCR5 Δ32 deletion would be proportional to the age of the deletion. Using a sample of 4000 individuals from 38 ethnic populations, Stephens et al. estimated that the CCR5-Δ32 deletion occurred 700 years ago (275-1875, 95% confidence interval). Another group, Libert et al. (1998), used microsatellite mutations to estimate the age of the CCR5 Δ32 mutation to be 2100 years (700-4800, 95% confidence interval). On the basis of observed recombination events, they estimated the age of the mutation to be 2250 years (900-4700, 95% confidence interval). A third hypothesis relies on the north-to-south gradient of allele frequency in Europe, which shows that the highest allele frequency occurred in the Nordic countries and lowest allele frequency in southern Europe. Because the Vikings historically occupied these countries, it may be possible that the allele spread throughout Europe due to the Viking dispersal in the 8th to 10th centuries. Vikings were later replaced by the Varangians in Russia, which may have contributed to the observed east-to-west cline of allele frequency.