A DNA molecule is more than just a long, monotonous string of nucleotides. Instead, it is divided into functional units known as genes. All cells use information encoded in their DNA to control or regulate protein synthesis. Gene expression refers to the process of turning on a gene to produce RNA and protein. All known life— eukaryotes (including multicellular organisms), prokaryotes (bacteria and archaea), and viruses—use gene expression to generate the macromolecular machinery for life. Gene expression is the most fundamental level at which the genotype gives rise to t in genetics. The regulation of gene expression is a highly complex process. Strictly speaking, "gene expression" refers to the process of a gene being activated until a mature protein is found in its corresponding compartment to perform its function and contribute to the expression of a cell's phenotype. The purpose of the expression studies is to detect and quantify messenger RNA (mRNA) levels of a specific gene. The rapid activation of gene expression in response to stimuli is largely controlled by RNA polymerase II-dependent transcription. Gene expression microarrays are being used to study RNA expression and may be used to derive profiles/signatures associated with tumour and normal tissue radio response. The events that occur during the transcription cycle in eukaryotes that are important for the rapid and specific activation of gene expression in response to external stimuli are discussed in this review.