Entry of budding yeast cells into the mitotic cell cycle requires the activity of a conserved regulatory kinase encoded by the CDC28 gene. The kinase is thought to trigger entry into the cell cycle or START, through association with a number of regulatory subunits known as G1 cyclins. A number of genes whose transcription is dependent on CDC28 and thus linked to START are controlled by two transcription factors, SWI4 and SWI6. The genes controlled by SWI4 and SWI6 include two known G1 cyclins (CLN1 and CLN2), a putative new G1 cyclin (HCS26), and the HO gene whose product initiates cell type switching. SWI4 and SWI6 act through a repeated sequence element, SCB (SWI4,6-dependent cell cycle box), found 2-10 times in the upstream regulatory sequences of target genes. We have constructed a library of mutants in the SCB using doped oligonucleotide mutagenesis. All single base pair changes examined compromised the ability of the SCB to activate transcription in vivo. Analysis of the behaviour of the mutant SCBs in an in vitro DNA binding assay shows that the inability to activate transcription can be explained by reduced binding of SWI4 and SWI6 to the mutant SCBs. This analysis, together with a consideration of the SCBs found upstream of known SWI4,6-dependent genes, leads to the proposal of a revised consensus sequence for this important regulatory element.