GDF5 and Arthritis: Despite the decrease in arthritis severity in vivo and diminished cell proliferation after Yap KO in vitro (p=0.023) (online supplemental figure 5), extensive expansion of Gdf5-lineage cells in synovium during AIA was observed in both Yap WT (p<0.001) and Yap cKO mice (p<0.001, figure 5E).