The mating type locus with two alleles (MATa and MAT alpha) determines cell type in yeast by activating and repressing sets of cell-type-specific genes. The two genes at MAT alpha, alpha 1 and alpha 2, are transcribed divergently from a central promoter region. Deletions in this intergenic region have been used to map DNA sequences involved in the transcription and regulation of the MAT alpha genes. A single promoter region, essential for transcription of both alpha 1 and alpha 2, is found in the region between alpha 1 and alpha 2. Deletions removing the alpha 1 or alpha 2 TATA box are still transcribed but the transcripts fail to initiate properly. A separate regulatory region is also found between alpha 1 and alpha 2. Deletions of this region lead to the constitutive expression of these genes. These regulatory mutants synthesize alpha 1 mRNA in diploids, but this is not sufficient to activate the alpha-specific genes.