ACVR1B and osteogenesis imperfecta: Importantly, genes within the activin signalling pathway ACVR1B (P < 0.01) and ACVR1 (P < 0.01) that have previously been shown to be associated with type V OI and that encode bone morphogenic protein type 1 of the TGFβ superfamily were also increased in OI fibroblasts compared with control.