AbstractIn the framework of the European BIOTECH project for sequencing the Saccharomyces cerevisiae genome, we have determined the nucleotide sequence of the left part of the cosmid clone 232 and the cosmid clone 233 provided by F. Galibert (Rennes Cedex, France). We present here 33,099 base pairs of sequence derived from the left arm of chromosome X of strain S288C. This sequence reveals 17 open reading frames (ORFs) with more than 299 base pairs, including the published sequences for ARG3, LIGTR/LIG1, ORF2, ACT3 and SCP160. Two other ORFs showed similarity with S. cerevisiae genes: one with the CAN1 gene coding for an arginine permease, and one with genes encoding the family of transcriptional activators containing a fungal Zn(II)2‐Cys6 binuclear cluster domain like that found in Ppr1p or Gal4p. Both putative proteins contain a leucine zipper motif, the Can1p homologue has 12 putative membrane‐spanning domains and a putative α2‐SCB‐α2 binding site. In a diploid disruption mutant of ORF J0922 coding for the transcriptional activator homologue, no colonies appeared before 10 days after transformation and then grew slowly. In contrast, haploid disruption mutants showed a growth phenotype like wild‐type cells. One ORF showed weak similarity to the rad4 gene product of Schizosaccharomyces pombe and is essential for yeast growth. Five ORFs showed similarity to putative genes on the right arm of chromosome XI of S. cerevisiae. Two of them have similarity to each other and belong to a family of extracellular proteins that groups mammalian SCP/Tpx‐1, insects Ag3/Ag5, plants PR‐1 and fungi Sc7/Sc14. Three small ORFs are completely contained in the larger ORFs on the corresponding complementary strand and thus probably do not represent real genes. The DNA sequence has been deposited in the EMBL data library under Accession Number X83502.