<P> Analysis of expression is not limited to quantification; localisation can also be determined . mRNA can be detected with a suitably labelled complementary mRNA strand and protein can be detected via labelled antibodies . The probed sample is then observed by microscopy to identify where the mRNA or protein is . </P> <P> By replacing the gene with a new version fused to a green fluorescent protein (or similar) marker, expression may be directly quantified in live cells . This is done by imaging using a fluorescence microscope . It is very difficult to clone a GFP - fused protein into its native location in the genome without affecting expression levels so this method often cannot be used to measure endogenous gene expression . It is, however, widely used to measure the expression of a gene artificially introduced into the cell, for example via an expression vector . It is important to note that by fusing a target protein to a fluorescent reporter the protein's behavior, including its cellular localization and expression level, can be significantly changed . </P> <P> The enzyme - linked immunosorbent assay works by using antibodies immobilised on a microtiter plate to capture proteins of interest from samples added to the well . Using a detection antibody conjugated to an enzyme or fluorophore the quantity of bound protein can be accurately measured by fluorometric or colourimetric detection . The detection process is very similar to that of a Western blot, but by avoiding the gel steps more accurate quantification can be achieved . </P> <P> An expression system is a system specifically designed for the production of a gene product of choice . This is normally a protein although may also be RNA, such as tRNA or a ribozyme . An expression system consists of a gene, normally encoded by DNA, and the molecular machinery required to transcribe the DNA into mRNA and translate the mRNA into protein using the reagents provided . In the broadest sense this includes every living cell but the term is more normally used to refer to expression as a laboratory tool . An expression system is therefore often artificial in some manner . Expression systems are, however, a fundamentally natural process . Viruses are an excellent example where they replicate by using the host cell as an expression system for the viral proteins and genome . </P>

Which of the following hormones works by direct gene activation