Galactose triggers a signal that causes GAL4 to activate transcription of the GAL genes of galactose regulon which contain GAL1, GAL7 and GAL10 genes. Although these GAL genes are located close together, they do not comprise an operon; they are each transcribed into separate mRNA molecules. This signal is mediated by regulatory protein GAL80. So the GAL4 is an transcription activator, in contrast, GAL80 is thought to act as a negative regulator.

In the absence of galactose, GAL4 is inhibited by its interaction with GAL80. In the presence of galactose, however, the inhibitory action of GAL80 is releved, and GAL4 is free to activate the GAL genes. GAL4 activates transcription of the GAL genes of this regulon by binding to upstream activating sequence (UAS) located in the promoter of each gene in the regulon.