DNA microarray

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.