This lack of specificity has the potential to lead to unwanted side effects, such as the gingival bleeding and telangiectasias observed in patients treated with an ActRIIB-Fc fusion protein, where binding and inhibition of BMP9 activity is thought to be the cause of these adverse events or alterations in follicle stimulating hormones (likely due to activin A inhibition) in patients treated with an antibody directed against ActRIIB (54,59). The gene discussed is ACVR2B; the disease is telangiectasis.