Summary Synonymous mutations change the DNA sequence of a gene without affecting the amino acid sequence of the encoded protein. Although emerging evidence suggests that synonymous mutations can impact RNA splicing, translational efficiency, and mRNA stability 1 , studies in human genetics, mutagenesis screens, and other experiments and evolutionary analyses have repeatedly shown that most synonymous variants are neutral or only weakly deleterious, with some notable exceptions. In their recent article, Shen et al. claim to have disproved these well-established findings. They perform mutagenesis experiments in yeast and conclude that synonymous mutations frequently reduce fitness to the same extent as nonsynonymous mutations 2 . Based on their findings, the authors state that their results “imply that synonymous mutations are nearly as important as nonsynonymous mutations in causing disease.” An accompanying News and Views argues that “revising our expectations about synonymous mutations should expand our view of the genetic underpinnings of human health” 3 . Considering potential technical concerns with these experiments 4 and a large, coherent body of knowledge establishing the predominant neutrality of synonymous variants, we caution against interpreting this study in the context of human disease.

A minimal role for synonymous variation in human disease