This chapter presents a brief overview of how authors learned to manipulate DNA, identify genes, and produce many copies of any given nucleotide sequence in the laboratory. It discusses several approaches to exploring gene function, including new ways to monitor gene expression and to inactivate or modify genes in cells, animals, and plants. DNA cloning is one of the most important feats of recombinant DNA technology, as it is the starting point for understanding the function of any stretch of DNA within the genome. The production of recombinant DNA molecules in this way is a key step in the classical approach to DNA cloning. To introduce recombinant DNA into a bacterial cell, investigators take advantage of the fact that some bacteria naturally take up DNA molecules present in their surroundings. Using recombinant DNA techniques, a protein can be joined to a molecular tag, such as green fluorescent protein, which allows its movement to be tracked inside a cell.