Transcription of the HO gene of Saccharomyces cerevisiae, which encodes a site-specific endonuclease that initiates cell-type switching (reviewed in refs. 1,2), is restricted to a short window of the cell cycle in late G1 (refs 3,4). A repeated element in the upstream region of HO (the cell-cycle box, CCB) and two regulatory proteins, SWI4 and SWI6, are required for cell-cycle-dependent expression of HO. Biochemical experiments have identified a factor, CCBF (cell-cycle box factor), that binds to the CCB elements and that presumably plays a key part in cell-cycle regulation of HO. The SWI4 and SWI6 genes are required for formation of the CCBF-DNA complex. Here we report the nucleotide sequence of the SWI4 gene and show that it contains two copies of the conserved SWI6-cdc10 motif observed in SWI6 of budding yeast, the Schizosaccharomyces pombe cdc10 gene required for progression through G19, the Drosophila Notch gene, and in the Caenorhabditis elegans lin-12 and glp-1 genes. We demonstrate by using antibodies to the SWI4 protein in gel-shift assays that the protein is present in the CCBF-DNA complex.