BTD-4 has antimicrobial activity against the Gram-negative bacterium E.coli ML35, the Gram-positive bacterium S.aureus 502a, and the fungus C.albicans 16820. BTD-4 is a cyclic heterodimer composed of subunits A and C; disulfide-linked. Forms a cyclic peptide with subunit A (BTD-4). An additional intersubunit disulfide bond is formed. BTD-4, heterodimer, cyclized. Belongs to the alpha-defensin family. Theta subfamily.