Among a large collection of nonsense (termination) suppressors of Saccharomyces 
cerevisiae, a few remained obscure for their molecular nature. Of those, a group 
of weak and recessive suppressors, sup111, sup112 and sup113, is of particular 
interest because of their dependency on [PSI+], a yeast prion. From the facts 
that these suppressors map at positions quite similar to the UPF2, UPF3 and UPF1 
genes, respectively, and that some mutations in the UPF genes confer termination 
suppressor activity, we suspected that sup111, sup112 and sup113 would very well 
be mutant alleles of the UPF genes. We tested our speculation and found that 
sup113, sup111 and sup112 were in fact complemented with the wild-type alleles 
of UPF1, UPF2 and UPF3, respectively. We further obtained evidence that the 
UPF1, UPF2 and UPF3 loci of the strains carrying sup113, sup111 and sup112, 
respectively, had point mutations. From these results, we conclude that sup111, 
sup112 and sup113 are mutant alleles of UPF2, UPF3 and UPF1, respectively, and 
thus attribute suppressor activity of these mutations to defects in the NMD 
(nonsense-mediated mRNA decay) machinery.