Tubulin regulatory sequences drive expression of a tricistronic transcript that encodes three open reading frames, each separated by a self-cleaving P2A sequence (this results in separate translation of each protein). The first ORF encodes GAL4, the second ORF encodes Scer\GAL3, and the third ORF encodes GAL80. In the presence of galactose, the Scer\GAL3 protein has high affinity for GAL80, sequestering the GAL80 protein and preventing it from repressing GAL4-driven expression.