A DNA microarray is a multiplex technology used in molecular biology and in medicine. It consists of an arrayed series of thousands of microscopic spots of DNA oligonucleotides, called features, each containing picomoles (10⁻¹² moles) of a specific DNA sequence, known as probes (or reporters). This can be a short section of a gene or other DNA element that are used to hybridize a cDNA or cRNA sample (called target) under high-stringency conditions. Probe-target hybridization is usually detected and quantified by detection of fluorophore-, silver-, or chemiluminescence-labeled targets to determine relative abundance of nucleic acid sequences in the target. Since an array can contain tens of thousands of probes, a microarray experiment can accomplish many genetic tests in parallel. Therefore arrays have dramatically accelerated many types of investigation.

In standard microarrays, the probes are attached via surface engineering to a solid surface by a covalent bond to a chemical matrix (via epoxy-silane, amino-silane, lysine, polyacrylamide or others). The solid surface can be glass or a silicon chip, in which case they are commonly known as gene chip or colloquially Affy chip when an Affymetrix chip is used. Other microarray platforms, such as Illumina, use microscopic beads, instead of the large solid support. DNA arrays are different from other types of microarray only in that they either measure DNA or use DNA as part of its detection system.

DNA microarrays can be used to measure changes in expression levels, to detect single nucleotide polymorphisms (SNPs) , to genotype or resequence mutant genomes (see uses and types section). Microarrays also differ in fabrication, workings, accuracy, efficiency, and cost (see fabrication section). Additional factors for microarray experiments are the experimental design and the methods of analyzing the data (see Bioinformatics section).

MicroRNAs are a class of post-transcriptional regulators ^{[1] [2] [3]} . They are short ~22 nucleotide RNA sequences that bind to complementary sequences in the 3’ UTR of multiple target mRNAs, usually resulting in their silencing ^[4] . MicroRNAs target ~60% of all genes ^[5] , are abundantly present in all human cells ^[6] and are able to repress hundreds of targets each ^[7][8]. These features, coupled with their conservation in organisms ranging from the unicellular algae chlamydomonas reinhardtii ^[9] to mitochondria ^[10], suggest they are a vital part of genetic regulation with ancient origins ^[11].

MicroRNAs were first discovered in 1993 by Victor Ambros, Rosalind Lee and Rhonda Feinbaum during a study into development in the nematode C. elegans regarding the gene lin-14 ^[12]. This screen led to the discovery that the lin-14 was able to be regulated by a short RNA product from lin-4, a gene that transcribed a 61 nucleotide precursor that matured to a 22 nucleotide mature RNA which contained sequences partially complementary to multiple sequences in the 3’ UTR of the lin-14 mRNA. This complementarity was sufficient and necessary to inhibit the translation of lin-14 mRNA. Retrospectively, this was the first microRNA to be identified, though at the time Ambros et al speculated it to be a nematode idiosyncrasy. It was only in 2000 when let-7 was discovered to repress lin-41, lin-14, lin28, lin42 and daf12 mRNA during transition in developmental stages in c elegans and that this function was phylogenetically conserved in species beyond nematodes ^[13] [14], that it became apparent the short non-coding RNA identified in 1993 was part of a wider phenomenon. Since then over 4000 miRNAs have been discovered in all studied multicellular eukaryotes including mammals, fungi and plants. More than 700 miRNAs have so far been identified in humans ^[15] and over 800 more are predicted to exist. ^[16]. Comparing miRNAs between species can even be used to delineate molecular evolutionary history ^[17] on the basis that the complexity of an organism's phenotype may reflect that of the microRNA found in the genotype ^[18] .

Due to their abundant presence and far-reaching potential, miRNAs have all sorts of functions in physiology, from cell differentiation, proliferation, apoptosis ^[24] to the endocrine system ^[25][26], haematopoiesis ^[27], fat metabolism ^[28], limb morphogenesis ^[29] . They display different expression profiles from tissue to tissue ^[30] , reflecting the diversity in cellular phenotypes and as such suggest a role in tissue differentiation and maintenance ^[31] [32].