Standard building blocks of proteins ? closed loops of 25-30 amino acid residues ? have been recently discovered and further characterized by combined efforts of several laboratories. New challenging views on the protein structure, folding, and evolution are introduced by these studies. In particular, the role of van der Waals contacts in protein stability is better understood. They can be considered as locks closing the polypeptide chain returns and forming the loop-n-lock elements. The linearity of the arrangement of the standard loops in the proteins has important evolutionary implications. Selection pressure to maintain the loops of nearly standard size is reflected in the protein sequences as characteristic distance between hydrophobic residues, equal to the loop end-to-end distance. Further characterization of the loop-n-lock units reveals several sequence/structure prototypes, which suggests a new basis for protein classification. The following is a review of these studies.