Full length EST2 in a centromeric plasmid and the diploid S. cerevisiae EST2 deletion strain were generously provided by the Berman lab and have been previously described (25) . Individual mutant strains were obtained by tetrad dissection. Five potentially significant À1 RF signals were identified in the EST2 open reading frame using the Predicted Ribosomal Frameshift Database (17) . The wobble bases of five slippery heptamers were mutagenized to synonymous codons by oligonucleotide site-directed mutagenesis using the QuickChange II XL Site-Directed Mutagenesis Kit (Stratagene). Oligonucleotide design and reaction conditions were performed as recommended by the manufacturer with minor modifications. All mutations were confirmed by sequencing. Plasmids so generated are described in Supplementary Table S3 .