Proteins are polymers of amino acid building blocks that are joined by covalent peptide bonds (proteins are also known as polypeptides). They play critical roles in all organisms, from acting as channels through which metabolites enter cells to converting nutrients into energy or regulating production of other proteins. Until recently, each polypeptide prescribed by a genetic message was thought to have a single purpose or function. As more amino acid sequences of proteins were determined in various organisms, and as more experiments were done with specific proteins, the one-protein–one-function assumption was called into question because it underestimated the ingenuity at work in the cell. Geneticists and biochemists continue to discover that many proteins have more than one purpose.