mRNA (messenger RNA) vaccines carry instructions for making a protein which mimics an infectious agent. When mRNA is delivered into a cell, the protein is made in a process called translation. The amount of protein translated is critical: too low, and the immune system will not be trained; too high, and it may cause unwanted side effects. Typically, mRNAs have a “cap” structure to signal that it should be translated. Caps allow for high protein production, but are difficult to control. IRESes (internal ribosomal entry sites) are unique RNA sequences derived from viruses which allow translation without a cap. Different IRESes produce varying amounts of proteins and have unique characteristics in different cell types. Therefore, IRESes allow us to control the protein production from mRNA. IRESes can also be placed anywhere within the mRNA sequence, whereas the cap can only be on the end of the mRNA molecule. Therefore, IRESes allow translation from unique RNAs like bicistronic mRNAs encoding multiple proteins, or circular mRNAs which are highly stable compared to linear mRNAs. However, one major shortcoming of IRESes is that their translation efficiency is much lower than that of traditional cap-based translation. Therefore, in our study, we aim to: 1) characterize several IRES sequences to find IRESes with varying levels of translational efficiency, and 2) design synthetic IRESes with increased efficiency over natural IRESes.