Estimates of the size and copy number of the retrotransposons in a 240 kb region flanking the adhl gene of maize suggest that 33–62% of the maize genome is composed of the high copy-number retrotransposons found in this region. An additional 16% of the maize genome is estimated to be composed of middle and low copy-number retrotransposons. The sorghum genome, which is more than three-fold smaller than the maize genome, does not have any detected copies of the maize retrotransposons in a region orthologous to that of maize adhl. Thus, it appears that retrotransposons have increased the size of the maize genome two- to five-fold since the divergence of maize and sorghum from a common ancestor about 16 million years ago.