BMP4 and age-related macular degeneration: Another potential mechanism by which the HTRA1rs11200638 A allele may increase AMD risk is its ability to bind to TGF-β family members and to inhibit signaling of TGF-β family proteins, such as bone morphogenetic protein 2 and bone morphogenetic protein 4), which have previously been reported to act as negative growth regulators in RPE [89,137].