Activin binds the Activin receptor composed of a type II receptor (ACVR2A/B) and a type I receptor, in this case ACVR1B (ALK4) (Attisano et al. 1996, Zhou et al. 2000). Activin appears to interact initially with the type II receptor component (Attisano et al. 1996). It is unclear if the type II and type I receptors are associated before binding Activin. Any of Activin A (INHBA:INHBA), Activin AB (INHBA:INHBB), and Activin B (INHBB:INHBB) can bind and signal via an activin receptor containing the ACVR1B (ALK4) type I receptor.