Proteins are large and complex molecules made of amino acids. Proteins can be studied by gel electrophoresis in two ways. One way is to take a mixture of proteins and separate them in the gel. The other way is to break down a single protein into smaller pieces. The smaller pieces can then be separated in the gel. Proteins can be positively or negatively charged. To separate proteins by size only, protein mixtures can be coated with a chemical called sodium dodecyl sulfate (SDS) to give all proteins a negative charge before putting the mixture into the gel.