Probe intensity distributions before (t = 0) and after (t = 17 cycles) washing: Thick and thin lines refer to PM- and MM-probes of chip A, respectively. The different panels show the distributions of: (a) all probes of the array (~6·105 probes), (b) probes predominantly hybridized with non-specific transcripts (2·105 probes) and (c) probes predominantly hybridized with specific transcripts (2·104 probes). Accordingly, about 4·105 probes are not considered because they are significantly hybridized by non-specific and specific transcripts as well. Panel d shows the respective distributions of set-averaged log-intensities. The vertical dotted line at the right indicates the maximum intensity M referring to complete saturation of the probe spots.