Despite its short length, a molecule consisting of 10 amino acids designed by a Japan-based team more closely resembles a protein than a peptide in structural and theoretical studies. In light of examining the tiny protein, the researchers propose that specific biophysical properties based upon how a molecule folds should be the measure of whether that molecule is considered a protein, without any rigid cutoffs with respect to size (J. Am. Chem. Soc., DOI: 10.1021/ja8030533). By rule, the cutoff between peptides and proteins is currently 50 amino acids. Shinya Honda and colleagues at the National Institute of Advanced Industrial Science & Technology (AIST), in Tsukuba, synthesized the 10-residue molecule, called CLN025. X-ray crystal structure information was consistent with NMR studies in solution, showing that CLN025 has a well-defined 3-D structure. In addition, molecular dynamics simulations of CLN025's folding process indicate that the structure the AIST team observed is considerably more stable than other possible conformations. CLN025 may prove valuable for studying microscopic events in protein folding, says theoretical chemist Peter G. Wolynes of the University of California, San Diego.