Rat T lymphocyte expression examined by hybridisation of radioactive message to a nylon membrane containing 24,000 rat genes. Virtually all of the cells in the different tissues of an animal or of a plant carry exactly the same genes, despite their very different forms and functions. It's just that in muscle tissue, for example, the genes that are active are those required to make muscle cells, while in the brain, it is the genes for brain cells that are active. However, even in a specialised tissue or cell type not all of the genes are switched on all the time - indeed it is the interplay and orchestration of different patterns of gene activity that enables cells to carry out different functions at different times, and respond to different stimuli. Simply looking at the genome, i.e. the sequence of DNA in the cells of an organism, doesn't tell you which of the genes are switched on and which are not.
Expression profiling uses DNA arrays.
These are small solid supports such as nylon squares and microscope slides to which have been attached a large number of DNA fragments, each representing a single gene. These fragments are then tested for their ability to bind to message molecules extracted from cells and tagged with a fluorescent molecule. So when a fragment binds to its message, it can be assumed that the gene was active in the cell at the time the extract was taken. Moreover, intensity of fluorescent label bound to the fragment indicates how active the gene was. At Babraham, this approach is being used to study gene activity in T lymphocytes.