Ratios of signal intensity of PM to those of cognate MM probes with background cDNA. Ratios of signal intensity of PM to those of cognate MM random probes with background cDNA 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 averaged signal intensities were derived from signal intensities of all 150 probes of the same length. Standard deviations are given in additional file 6.