GAL80 is a regulatory protein found in yeast, specifically in the Saccharomyces cerevisiae species. It plays a crucial role in the GAL gene expression system, which is responsible for the metabolism of galactose. By inhibiting the activity of the GAL4 protein, GAL80 prevents the transcription of genes needed for galactose utilization when glucose is present. When glucose levels drop, GAL80 is inactivated, allowing GAL4 to activate the transcription of the GAL genes. This process ensures that yeast cells efficiently switch to using galactose as an energy source when preferred sugars are not available.