For the RNA-based microarray study, specific samples were subjected to dye-swap according to the experimental design, and data with consistently low probe intensity level – flagged as absent in all replicate samples – were discarded.
Low-level analysis which converts probe level data to a gene level expression data was done using robust multiarray average (RMA), implemented using the rma function of the Affymetrix package of the Bioconductor project in the R programming language [ 78- 80].
The two contigs with different expression profiles showed very low gene expression values, close to or below background level, suggesting a low probe efficiency or a low expression in the different tissues.
Low probe velocities minimize viscous losses.
To include probes representing genes that are expressed at a low level, probes whose expression was low but detectable (signal value of 50 or greater) was compared with those probes that were 'absent' in the treatment or the baseline hybridization.
This may be attributable to the cross-hybridization of microarray probes, low expression level in native plants, or unsuitable primer design.
Upon conducting low-level and probe set-level QC, it was clear that one of the met1 mex/mex 42 DPH GeneChips was aberrant.
Indeed, the MLPA assay of Bergval et al. [ 5] included 47 informative probe pairs (high multiplex level) and hybridisation occurred at a concentration of 0.25 nM of each probe, whereas a concentration of 5 nM of each probe gave high signal-to-noise ratios for positive results in our initial MOL-PCR tests with 6 probe pairs (low multiplex level) (data not shown).
As expected, probe sets with low expression level (low signal) were less reproducible in comparisons between the different sample groups, as were probe sets with very low fold-changes.
However, we were unable to directly measure low levels of probe remaining bound to the bead.
Without a target, both probes exhibited a low background level comparable to the buffer solution.
