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 Sup351–55 protein, produced from the sup35-240 allele, is included in Sup35 aggregates and destabilize them. Alternatively, single molecules of Sup351–55 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.