Table 1. Most extracellular filaments that use related extension ATPases to power extension do not have an antagonistic retraction ATPase.

Extracellular filament type	Extension ATPase	Predicted retraction/ evidence of retraction	Retraction ATPase present?
Type II secretion	GpsE	Yes	No
Gram-positive competence pili	ComGA	Yes	No
Type IVa pili	PilB	Yes	Yes
Type IVb pili	Variable depending on system	Yes	Sometimes
Type IVc tad pili	CpaF	Yes	No