Estimated duplication times (±S.E.) of pairs of paralogous genes on human Hox-bearing chromosomes, plotted on a timeline illustrating divergence times of major vertebrate taxa (Kumar and Hedges 1998). Only families for which the hypothesis of a molecular clock could not be rejected were included. The time window within which the vertebrate Hox clusters are believed to have duplicated is indicated by shading. Names of gene families are abbreviated as in Table 1.