Synonymous mutations in yeast can be harmful, but not as much as nonsynonymous mutations. Some scientists think these harmful effects are due to mistakes in the lab, not the mutations themselves. However, this idea doesn't match up with previous studies. Other researchers argue that comparing these mutations is unfair and that the frequency of these mutations in humans can be explained by other factors. Overall, the fitness effects of yeast synonymous mutations challenge the idea that they are mostly harmless.