Signal intensities of probes with background cDNA. Signal intensities of random probes with background cDNA generated from E. coli total RNA as a function of probe length in the 7 levels of target oligodeoxyribonucleotide concentration range of 1.4 fM to 1.4 nM. The signal intensities were determined using GCOS 1.0 software (Affymetrix). The signal intensities of one of the three PM probes and those of MM probes are represented by the averages of the mismatch probes the center bases of which were substituted with all other nucleotides. The signal intensities are plotted on a log(10)-scale. Standard deviations are given in additional file 5.