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.^[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.

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.^[ref]

The 1995 experiment used robotic printing of Complementary DNA onto glass, allowing many tests in a small 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.