We have sequenced on both strands a 40,257 bp fragment located near the left telomere of chromosome X of Saccharomyces cerevisiae. The sequenced segment contains 21 open reading frames (ORFs) at least 100 amino acids long. Five of the ORFs correspond to known amino acid sequences: two hypothetical proteins in the subtelomeric Y' repeat region of 65.4 and 12.8 KDa, the cytochrome B pre-mRNA processing CBP1 protein, the mitochondrial nuclease NUC1 and the CRT1 protein. Of the 16 remaining ORFs, eight show highest homologies with the S. cerevisiae hexose transporters family (two ORFs), the yeast alpha-glucosidase (two ORFs), the yeast PEP1 precursor, the Escherichia coli galactoside O-acetyltransferase, the S. cerevisiae 137.7 KDa protein located in the Y' region and a protein of unknown function of Schizosaccharomyces pombe. Finally, eight of the ORFs exhibit no significant similarity with any amino acid sequences described in data banks. DNA sequence comparison has revealed the presence of different repeated elements characteristic of yeast chromosome ends. Disruption studies have been performed on two ORFs encoding putative proteins of unknown function.