Refers to the level at which each of a chosen set of cellular genes is expressed, measured as mRNA transcript abundance. To determine an expression profile, messenger-RNA from relevant cells is copied into labeled (with fluorescence dye or radioactive nucleotide) complementary cDNA probes. The probes are then incubated on a DNA array containing spots of DNA from known genes being investigated. Hybridization of a given probe signifies the presence of the mRNA in question in the cell or tissue studied, i.e., the gene is active. The more an individual mRNA is abundant, the more intense the hybridization signal will be. The method allows in one assay the estimation of the activity of many genes. The first array was developed in the early 1980s as an “Oncochip” for profiling of oncogene expression.