In eukaryotes, mRNAs are monitored for errors in gene expression by RNA surveillance where untranslatable mRNAs are selectively degraded by the nonsense-mediated mRNA decay (NMD) pathway. Depending on the organism, three to seven genes are required for NMD. Besides RNA surveillance, the genes required for NMD serve a second purpose by controlling the overall abundance of a substantial fraction of the transcriptome.