GDF2 and osteoporosis: Femoral tissues of 4-week-old WT or BMP9-KO mice were subjected to micro-CT analysis as shown in Figure 1A. The BMP9-KO mice displayed osteoporosis characterized by reduced bone volume to tissue volume ratio (BV/TV), decreased trabecular number (Tb.N) and trabecular thickness (Tb.Th), and increased trabecular separation (Tb.Sp) when compared with WT mice (Fig. 1B).