cas2 mutant was isolated from a genetic screening for yeast mutant defective in protein folding in the cytosol based on the following criteria: temperature-sensitive lethality, loss of cytosolic enzymatic activity but with normal transcription and translation. A 4.8 kb DNA fragment containing PET18 complex was able to complement the ts lethal phenotype of cas2 mutants. Like pet18 mutant, cas2 mutant also bears respiratory deficiency and loss of cytosolic ds RNA killer virus. Further characterization showed that cas2 also exhibit accumulation of mitochondiral precursor and failure to grow on galactose and raffinose containing media. Despite that cas2 and pet18 have large deletions expanding several genes including MAK31 and MAK32 ORFs, the DNA fragments that rescue cas2 and pet18 are different. An ORF encoding a 78 amino acid protein which complements cas2 ts phenotype but not the mitochondirall deficiency lies between MAK31' and HSP30.. Disruption of this ORF also exhibits ts phenotype.