In 1984 a high molecular mass multisubunit protease complex was isolated from Saccharomyces cerevisiae [Achstetter et al. 1984] which proved to be the yeast homologue of the 20S proteasome complexes found in all eukaryotic cells [Kleinschmidt et al. 1988]. The yeast 20S proteasome is able to cleave chromo- and fluorogenic peptides at the carboxyterminus of hydrophobic, basic or acidic amino acids (chymotrypsin-like-, trypsin-like- and peptidyl-glutamyl-peptide hydrolyzing activity, respectively) [Heinemeyer et al. 1991]. The yeast 20S proteasome is composed of different subunits, showing a set of protein bands in the SDS-PAGE with molecular masses ranging from 20 to 35 kDa. They can be separated into 14 protein spots after two-dimensional gel electrophoresis [Heinemeyer et al. 1991]. Genes named Y7, Y13, PRS1 and PRS2 (independendly cloned as Y8 and SCL1) were cloned and sequenced on the basis of protein sequences of 20S proteasome subunits, genes named PRS3, PUP1, PUP2 and PUP3 were sequenced by chance [for summary see Hilt et al. 1993b]. We cloned the s-type genes PREI, PRE2, PRE3 and PRE4 by complementation of mutants defective in the chymotrypsin-like- (prel and pre2 mutants) or the PGPH-activity (pre3 and pre4 mutants) of the proteasome [Heinemeyer et al. 1991, Heinemeyer et al. 1993, Hilt et al. 1993a, Enenkel et al. 1994]. Additionally we cloned two α-type genes PRE5 and PRE6 using peptide sequences derived from purified proteasome subunits, extending the number of yeast 20S proteasome subunit genes to 14 [Heinemeyer et al. 1994].