GAL4 is a transcription factor commonly found in yeast, specifically Saccharomyces cerevisiae. It plays a crucial role in the regulation of genes involved in the metabolism of galactose, a type of sugar. When galactose is present, GAL4 binds to specific DNA sequences, activating the expression of genes that help the yeast utilize this sugar for energy. In research, GAL4 is often used as a tool in molecular biology. Scientists can manipulate GAL4 to control gene expression in various organisms, including Drosophila (fruit flies) and mice. This ability makes it a valuable component in studies of gene function and regulation.