Microarray platform comparison. Series of expression profiling assays were performed using the HG-U133A 2.0 Affymetrix GeneChip (3 replicates per RNA sample), the human oligonucleotide Operon set (4 replicates) and the Actichip arrays (10 replicates). Experiments were carried out using the same lots of RNAs extracted from human carcinoma MCF-7 cell line and from human skeletal muscle. Data were analysed as stated in the "Methods" section resulting in two groups of genes : "detected" and "not detected". The genes found similarly expressed or not expressed in less than 2/3 of the replicated assays were deemed "non reproducible". The histograms show the distribution of the three groups of genes for each platform relative to the samples. Results are expressed as percentages relative to the total number of genes simultaneously represented on each array platform.