Proteins are linear chains of amino acids that fold into complex three-dimensional structures associated with particular functions. Extensive structural analyses of proteins by X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, and cryo-electron microscopy have revealed that many proteins are modular, and consist of domains that fold independently. If an amino acid sequence folds into a particular structure, it is a safe bet that a similar sequence in a second protein will adopt a similar structure. Hundreds of unique modules have been defined and archived in databases that allow prediction of structure and function on the basis of amino acid sequence. However, sequences still exist for which structure cannot be predicted. Such was the case for myeloid-derived growth factor (MYDGF), a protein of ~145 amino acids present in nearly every tissue and cell in the human body and in organisms as distant from humans as slime molds.