= Microarray
A <microarray> arranges many small test sites at known positions on a surface so their signals can be measured in parallel. In a <DNA microarray>, each site carries a known nucleic-acid sequence; <DNA hybridization> between that sequence and a labelled sample provides a readout of complementary material in the sample.
A classic expression experiment printed <Complementary DNA> at high density on glass and hybridized labelled material derived from cellular <messenger RNA>. Many <genes> could thus be compared in one small assay instead of one reaction at a time.<Quantitative monitoring of gene expression patterns with a complementary DNA microarray by M. Schena and others>[1995, abstract: robotic cDNA printing, glass substrate and parallel expression measurement]{ref}
A twofold difference in signal can suggest different transcript abundance after background correction and normalization. It is not automatically a twofold difference in <protein> abundance: probe binding, labelling and the steps between <transcription> and <biological translation> affect the correspondence.
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