Microarrays - In case of microarrays, a series of probes are immobilized on a glass slide as microdots, which are then hybridized with a mixture of test DNA sequences that are labelled with a fluorochrome.

An extremely large number of probes is spotted on to the slide, each probe is a pure preparation, the test DNA is a mixture of sequences and the results are visualized by confocal microscopy.

Microarrays were first used in case of yeast, which has 6,000 genes. Every yeast gene was obtained as an individual clone and a sample of each gene was spotted onto glass sides in arrays of 80 spots x 80 spots. In order to determine the identity of genes active in yeast cells under a set of given conditions, mRNA is extracted from these cells.