Messenger RNA (mRNA) is a type of molecule that carries genetic instructions from DNA to the parts of the cell that produce proteins. However, mRNA molecules can sometimes stick together, forming clumps or aggregates, which can be harmful to cells. Normally, cells contain thousands of mRNA molecules in a very small space, yet these molecules rarely form large aggregates. This is important because unwanted interactions between mRNAs can prevent them from being used to make proteins, and large aggregates can be toxic to the cell. Scientists have long wondered how cells manage to avoid this problem. A team of researchers from the Whitehead Institute has uncovered a possible explanation. Their study suggests that evolution has shaped the sequences of mRNA in a way that reduces the likelihood of unwanted interactions between mRNA molecules. Led by Ankur Jain and Marco Todisco, the researchers found that DNA sequences not only need to encode functional proteins but also must produce mRNAs that remain soluble within the cell. This adds a new layer of complexity to how genetic sequences evolve. To test their hypothesis, the researchers used Escherichia coli (E. coli), a well-studied bacterium. Using computer simulations, they modeled how mRNA molecules behave at concentrations similar to those found inside cells. Based on the physical properties of RNA alone, the simulations predicted that the molecules should clump together, especially longer mRNAs. When they tested purified mRNA outside of the cell, they found that it indeed formed aggregates, confirming the simulations. The researchers discovered that the solution to RNA stickiness lies within the genetic code itself. DNA uses three-letter sequences, called codons, to encode proteins, and most amino acids can be encoded by multiple codons. This allows for flexibility in mRNA sequences. By creating alternative versions of E. coli mRNAs that still produced the same proteins, the researchers found that naturally occurring sequences were less likely to interact with each other. These natural mRNAs tended to fold in a way that hid potentially sticky parts, reducing the chance of unwanted interactions. The researchers also found similar patterns in human mRNAs, suggesting that this phenomenon is not limited to bacteria. This adds a new perspective to the "central dogma" of molecular biology, which describes the flow of genetic information from DNA to RNA to protein. The study implies that evolution must also account for the physical behavior of mRNA molecules. While purified mRNA clumps easily, this doesn't happen in healthy cells, suggesting that other cellular components may help prevent unwanted RNA interactions. These findings could influence the development of mRNA-based therapies, such as vaccines. When designing synthetic mRNAs, researchers often choose between different codons that produce the same protein. The new study suggests that how the RNA folds and interacts with other RNAs should also be considered when selecting these sequences. This adds another important factor to the design of effective mRNA therapeutics.