Periodicity plots for the four analyzed genomes. The ordinate displays the normalized intensity Q*(P) of the periodic signal in the spacing of AA and TT dinucleotides for the period shown by the abscissa. The parameters smin and smax were set at 30 and 100 bp, respectively. See text for details. The horizontal lines and shading refer to statistical significance of the peaks in the plot. The dark shaded area corresponds to values below the 50th percentile of the dominant periodic signal in random sequences. The light shaded area refers to values between the 50th and 95th percentiles. Peaks rising above the shaded area can be considered statistically significant. An additional line without shading refers to the 99th percentile. The definition of the MaxQ and PMaxQ indices is demonstrated in the periodicity plot for M. jannaschii.