 It requires BMP signaling for its expression

FKBP12 binds the GS region of the type I receptor preventing phosphorylation of the receptor by the type II receptors.It is believed that FKBP12 and its homologs help to prevent type I receptor activation in the absence of a ligands, since ligand binding causes its dissociation.There are two other SMADs which complete the SMAD family, the inhibitory SMADs (I-SMADS), SMAD6 and SMAD7.