The study of the structure and function of genes - in particular, how alterations in a particular gene affect biological behavior. Using "forward genetics", investigators isolate cells or organisms with defects in a particular behavior and then discover which gene is responsible; using "reverse genetics", investigators modify a gene of interest and then test the impact on the biological behavior of the cell or organism. Numerous investigators here are using this approach to study bacteria, viruses, yeast, mammalian cells, mice and humans - in order to decipher how a broad range of biological strategies are genetically programmed.