= DNA microarray
{c}
A <DNA microarray> is a <microarray> whose spatially indexed sites carry known <DNA> sequences. A labelled sample binds through <DNA hybridization>, and signal at each site reports complementary material retained there. In a <gene expression> assay, material derived from <messenger RNA> is compared across many <genes> in parallel; the readout depends on transcript abundance and the chemistry of binding and labelling.<Quantitative monitoring of gene expression patterns with a complementary DNA microarray by M. Schena and others>[1995, abstract: printed cDNA, hybridization and expression measurements]{ref}
The 1995 experiment used robotic printing of <Complementary DNA> onto glass, allowing many tests in a small hybridization volume.<Quantitative monitoring of gene expression patterns with a complementary DNA microarray by M. Schena and others>[Abstract: glass substrate and two-microlitre hybridization volume]{ref}
For a synthetic calculation, suppose the two sample signals are 1,000 and 600 units, with background 200 units in each. Background-corrected signals are 800 and 400, giving a ratio of $2$ and a base-two <logarithm> of $1$. The raw ratio $1000/600$ would give a different answer. Further correction for sample-wide effects may still be required. A transcript ratio supports an interpretation about <gene expression>; deriving a <protein> abundance ratio requires additional evidence about <biological translation> and turnover.
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