These cyclic peptides all contain four amino acids. One of the amino acids is either proline or the homolog pipecolic acid. All contain Aeo (2-amino-9,10-epoxi-8-oxodecanoic acid) except apicidin, which lacks the epoxide group.

All are made by filamentous fungi. HC-toxin is made by the maize pathogen Cochliobolus carbonum; Cyl-2 is made by the plant pathogen Cylindrocladium scoparium (now known as Calonectria pteridis); WF-3161 is made by the saprophyte Petriella guttulata; trapoxin A is made by the saprophyte Helicoma ambiens; chlamydocin by the saprophyte (perhaps nematode parasite?) Diheterospora chlamydosporia; and apicidin by an unidentified Fusarium species.

All, including apicidin, are potent inhibitors of histone deacetylase (HDAC). An affinity column based on trapoxin was used for the first isolation of a histone deacetylase (Taunton, Hassig, and Schreiber, Science (1996) 272:408), starting the 'modern era' of HDAC research.