Distribution of the number of conserved genes within colinear regions of Arabidopsis and rice. The black, gray, and white histograms show the distribution of the blocks emerged by maximally 100%, 5%, and 1% chance, or 0%, 95%, and 99% significance levels, respectively. We propose to use the 99% significance level (i.e., maximally 1% probability to be generated by chance) as default setting.