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Expansion of a Glutamine (Gln) repeat above a specific critical size in certain proteins gives rise to aggregation-prone proteins that cause neurodegenerative disorders, such as Huntington's disease. However, proteins with long hydrophilic polyglutamine repeats are more frequently found in nature than proteins with long homogeneous repeats of other amino acids, such as hydrophobic (Ala)(n) and (Leu)(n). To explore this finding, the effects of expression in mammalian cells of polyglutamine and polyleucine encoded by mixed DNA repeats were compared. It was found that polyleucine is significantly more toxic than polyglutamine. In addition, we show that polyleucine stretches display a high propensity for aggregation utilizing two complementary biochemical assays and that polyleucine stretches can also be detected by the monoclonal antibody 1C2, which specifically recognizes expanded pathogenic and aggregation-prone glutamine repeats. Together, these results suggest that polyglutamine stretches are in fact relatively well tolerated and that nature may select more strongly against DNA stretches that encode long hydrophobic homopolymeric amino acid stretches, such as polyleucine -- possibly owing to their strong propensity for aggregation. In keeping with this notion, an increasing number of diseases are found to be associated with expansion of stretches of hydrophobic amino acids, including oculopharyngeal muscular dystrophy (OPMD), which is associated with expansion of a hydrophobic polyalanine stretch.