Variation in the regulation of gene transcription between individuals is thought to be a major cause of phenotypic diversity. Here, individual differences in the binding of transcription-factor proteins are studied. A well-known transcription factor in the yeast pheromone pathway is used as an example, and the underlying genetic loci responsible for variation in its binding are mapped. The study reveals new insights into the mechanisms of gene regulation, and new regulators of the yeast pheromone pathway