Average size of isochores belonging in the five isochore families for all the vertebrates tested. The data for human, fishes  and chicken  are reported for the sake of comparison. A horizontal guideline at 0.9 Mb correspond to the average size of isochores in the human genome . A vertical line is drawn to divide mammals and chicken from the fishes. Asterisks refer to sizes that are probably overestimated (see Text and Table 1).