Comparison of the spatial distribution of repeats for (a) an imprinted gene that is not in a cluster ( WT1 ) and (b) an imprinted cluster ( IGF2R ) across the species examined in our analysis. The repeat element distribution is shown as color-coded vertical lines in tracks for each species. There is a dramatic change in the number and spatial distribution of repeats between eutherians and platypus. There is also an increase in the number and size of CpG islands throughout the regions examined. Large-scale accumulation of repeats appears to have occurred after the bird-mammal divergence, with significantly fewer repeats of all classes seen in the chicken. The repeat distribution across an additional five regions of various sizes can be viewed in Additional data file 1. LINE, long interspersed nuclear element.