[2][3] θ-defensins are cyclic peptides of 18 amino acids (~2 kDa), possessing antimicrobial activity against a range of Gram-positive and Gram-negative bacteria, fungi, and some retroviruses.

[4][5][6][7] They consist of a pair of antiparallel β-sheets linked by three disulfide bonds arranged as a ladder along the sheets to form an extremely stable structure.

[8] In rhesus macaque (Macaca mulatta) and olive baboon (Papio anubis), θ-defensins are produced from precursor proteins with 76 amino acids each.

Antiviral activity appears to derive from their binding to the sugar component of glycoproteins and blocking the entry of viruses into the cell.

[10] When the mature defensin that would be expressed is chemically synthesised in a laboratory, it shows antimicrobial activity (including anti-retrovirus, leading to the name "retrocyclin").