GAL3 is a gene found in the yeast Saccharomyces cerevisiae that plays a crucial role in the regulation of galactose metabolism. It encodes a protein that acts as a sensor for the presence of galactose, a type of sugar. When galactose is available, GAL3 activates the expression of other genes involved in its metabolism, allowing the yeast to utilize this sugar for energy. The function of GAL3 is part of a larger regulatory system that includes other genes like GAL1 and GAL7. Together, these genes ensure that yeast can efficiently switch to using galactose when it is present in the environment, demonstrating the organism's adaptability to different nutrient sources.