Translation of messenger RNAs (mRNAs) with premature termination codons produces truncated proteins with potentially deleterious effects. This is prevented by nonsense-mediated mRNA decay (NMD) of these mRNAs. NMD is triggered by ribosomes terminating upstream of a splice site marked by an exon-junction complex (EJC), but also acts on many mRNAs lacking a splice junction after their termination codon. We developed a genome-wide CRISPR flow cytometry screen to identify regulators of mRNAs with premature termination codons in K562 cells. This screen recovered essentially all core NMD factors and suggested a role for EJC factors in degradation of PTCs without downstream splicing. Among the strongest hits were the translational repressors GIGYF2 and EIF4E2. GIGYF2 and EIF4E2 mediate translational repression but not mRNA decay of a subset of NMD targets and interact with NMD factors genetically and physically. Our results suggest a model wherein recognition of a stop codon as premature can lead to its translational repression through GIGYF2 and EIF4E2. Author summary: A premature stop codon–a signal to abort production of a protein before the normal end has been reached—is treated differently by the cell than the corresponding normal stop codon. Many disease-causing mutations result in premature stop codons, and current research suggests that differences between stop codons could be exploited for clinical therapy. We have carried out a genetic screen to identify factors that regulate the production of protein from an messenger RNA (mRNA) with a premature stop codon. Our screen recovered previously known factors in this process, as well as unexpected roles for factors that normally mark the sites of RNA splicing, as well as a pair of proteins (GIGYF2 and EIF4E2) that mediate translational repression but not mRNA decay of mRNAs with a premature stop codon. [ABSTRACT FROM AUTHOR]