Upper panel: Δ-vs-Σ trajectories of six selected probe-pairs taken from the HG-U133 spiked-in experiment (Affymetrx). The sequences of the PM-probes are shown in the insertion below. The probe-pairs are numbered with increasing C and decreasing A content of the respective PM-probe (see of insertion on the right). The symbols are the experimental data. The lines are calculated using Eq. (10). Lower panel: Variability of the Δ-vs-Σ trajectories of all 498 spiked-in probes. The boxplot indicates the scattering of the start- and maximum-coordinates for R = 0 and R = Rmax, respectively. The Δ-vs-Σ trajectories are averaged over all probe-pairs with the middle base A, T, G and C of the PM, respectively. The variability due to the middle base refers to the 75%/25% quantiles of the distribution of the individual probe trajectories. The consideration of the nearest neighbours in terms of the middle triple extends this range: only the trajectories with the largest and smallest maximum value (for middle triples CCC and CGC, respectively) are shown (see also ref. ).