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The essential SUP35 gene encodes yeast translation termination factor eRF3. Previously, we isolated nonsense mutations sup35-n and proposed that the viability of such mutants can be explained by readthrough of the premature stop codon. Such mutations, as well as the prion [ PSI + ], can appear in natural yeast populations, and their combinations may have different effects on the cells. Here, we analyze the effects of the compatibility of sup35-n mutations with the [ PSI + ] prion in haploid and diploid cells. We demonstrated that sup35-n mutations are incompatible with the [ PSI + ] prion, leading to lethality of sup35-n [ PSI + ] haploid cells. In diploid cells the compatibility of [ PSI + ] with sup35-n depends on how the corresponding diploid was obtained. Nonsense mutations sup35-21 , sup35-74, and sup35-218 are compatible with the [ PSI + ] prion in diploid strains, but affect [ PSI + ] properties and lead to the formation of new prion variant. The only mutation that could replace the SUP35 wild-type allele in both haploid and diploid [ PSI + ] strains, sup35-240 , led to the prion loss. Possibly, short Sup35 1-55 protein, produced from the sup35-240 allele, is included in Sup35 aggregates and destabilize them. Alternatively, single molecules of Sup35 1-55 can stick to aggregate ends, and thus interrupt the fibril growth. Thus, we can conclude that sup35-240 mutation prevents [ PSI + ] propagation and can be considered as a new pnm mutation.