The mating type locus with two alleles (MATa and MATα) determines cell type in yeast by activating and repressing sets of cell-type-specific genes. The two genes at MATα, α1 and α2, 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 MATα genes. A single promoter region, essential for transcription of both α1 and α2, is found in the region between α1 and α2. Deletions removing the α1 or α2 TATA box are still transcribed but the transcripts fail to initiate properly. A separate regulatory region is also found between α1 and α2. Deletions of this region lead to the constitutive expression of these genes. These regulatory mutants synthesize α1 mRNA in diploids, but this is not sufficient to activate the α-specific genes.