From wikipedia:

Diketopiperazines (DKP) are a class of cyclic organic compounds that result from peptide bonds between two amino acids to form a lactam. They are the smallest possible cyclic peptides.
A diketopiperazine was the first peptide to have its complete three-dimensional structure described, in work undertaken at Caltech by Robert Corey in the 1930s. Corey studied the cyclic anhydride of the dipeptide glycyl glycine.
Diketopiperazines are commonly biosynthesized from amino acids by different organisms, including mammals, and are considered to be secondary metabolites. Some protease enzymes, such as dipeptidyl peptidases, cleave the terminal ends of proteins to generate dipeptides which naturally cyclize to form diketopiperazines.
Due to their rigid structure, chiral nature, and varied side chains, diketopiperazines are an attractive scaffold for drug design. Both natural and synthetic diketopiperazines have a wide variety of biological activities including antitumor, antiviral, antifungal and antibacterial activities.