In an effort to understanding the mechanism of pre-mRNA splicing, we had previously demonstrated that SPP2 ( suppressor of pre-mRNA processing) gene product of yeast Saccharomyces cerevisiae is involved in the pre-mRNA splicing by showing that inhibition of SPP2 gene expression results in the accumulation of pre-mRNA. To investigate a role of the SPP2 product in the splicing pathway, we attempted to isolate temperature-sensitive (ts) spp2 mutations. First, we have constructed a mutant spp2 library (pHIS-spp2 library) by treating the cloned SPP2 structural gene with a mutagen, methoxyamine, followed by subcloning the mutated DNA into an unmutagenized plasmid. Using the pHIS-spp2 library, we have transformed a haploid yeast strain (Mata his3 SPP2::LEU2/YCp50-SPP2) to his+ and selected 5-FOA (5-fluoro-orotic acid) resistant cells to cure the YCp50-SPP2 helper plasmed. Among about 3,000 spp2:: LEU2/pHIS-spp2 strains obtained, 6 temperature-sensitive (sensitive at 38。C) spp2 mutants have been identified.