GDF5 and Osteopenia: A mutant GDF5 protein called BB-1, which carries two point mutations in the BMP type1 receptor (BMPR-I) binding site of GDF-5 (and thus shows increased affinity to the subunit BMP receptor type 1A (BMPR-IA) [43]), also significantly enhanced the bone formation in a sheep lumbar osteopenia model for at least 3 months after a single therapeutic application [11].