The HO gene, which encodes an endonuclease responsible for initiating mating type switching in yeast, is transcribed at START during the cell cycle of mother cells but not at all during the cell cycle of daughter cells. At least six genes, called SWI1-6, are necessary for HO transcription. We describe the isolation and characterization of mutations in two new genes called SDI1 and SDI2, which partially suppress the requirement for SWI5 and which cause daughter cells to express HO. The analysis of mating type switching in swi5- sdi1- and SWI5+ sdi1- strains suggests that the mother cell specificity of HO transcription is due exclusively to the selective action of SWI5 in mother cells. SDI1 encodes (or regulates) a repressor protein that binds to the HO promoter and prevents HO transcription in daughter cells by causing HO to be fully SWI5 dependent.