All proteins of Saccharomyces cerevisiae have been compared to determine how frequently segments from one protein are present in other proteins. Proteins that are recently evolutionarily related were excluded. The most frequently present protein segments are long, tandem repetitions of a single amino acid. For some of these segments, up to 14% of all proteins in the genome were found to have similar peptides within them. These peptide segments may not be functional protein domains. Although they are the most common shared feature of yeast proteins, their ubiquity and simplicity argue that their probable function may be to simply serve as spacers between other protein motifs.