A cellular machine known as the ribosome builds proteins according to mRNA instructions. Ribosomes "read" the mRNA code and enlist other molecules to build the called-for protein from a store of amino acids free floating outside the nucleus. The ribosome links the corresponding amino acids together into a protein chain. A typical ribosome is made of at least 50 proteins and several RNA molecules. NSF-supported researchers are examining many of those components and hope someday to understand how each helps the ribosome build proteins.