Humans have been altering the genetic code of plants and animals for millennia by selectively breeding for desirable features. Scientists are searching for a deeper understanding of how life works by creating new systems to test the limits of biological knowledge. More recent advances have enabled scientists to make new sequences of DNA from scratch. It is the design and construction of new biological parts, devices and systems as well as the re-design of existing, natural biological systems for alternate, useful purposes that has come to be know as synthetic biology—or synbio. Scientists can now use computers and laboratory chemicals to design organisms that do new things—like produce biofuels or excrete the precursors of medical drugs. The tools of this branch of science include DNA sequencing, fabrication of genes, modeling how synthetic genes behave and precisely measuring gene behavior.