Introduction. An overview. Yeast is a eukaryotic model organism. A cell as a biochemical entity.- Adaptation to Environment. Growth and multiplication. Enzyme adaptation. Induction to Leloir enzymes.- Genetic Dissection of Galactose Metabolism. Genetic analysis of GAL regulon. Genetic mapping of GAL genes.- Genetic Analysis GAL Genetic Switch. Negative control by the repressor. Operator repressor model of GAL regulon. Genetic interactions. Conditional lethal mutations. Revised model of GAL genetic switch. Signal transduction in GAL regulon.- Molecular Genetics of GAL Regulon. Cloning: a perspective. Genomic organization of GAL cluster. Isolation of GA
L4: the transcriptional activator. Isolation of GA
L80: the repressor. Isolation of GA
L3: the signal transducer.- Signal Transduction Revisited. Revised model of signal transduction. Genetic dissection of signal transduction.- Versatile Galactose Genetic Switch. Transcription activation. Glucose repression. Fine regulation of GAL genetic switch.- Paradigmatic Role of Galactose Switch. GAL regulon and genomics. GAL regulon and systems biology. Galactose metabolism and evolution. GAL Switch as a tool. Lessons learned.