TGFB1 and Feingold syndrome type 1: Our findings strongly suggest that TGF-β signaling upregulation in mesenchymal progenitor cells plays a major causal role in the skeletal abnormalities of Feingold syndrome type 2, whereas downregulation of the PI3K signaling pathway significantly contributes to the pathogenesis of Feingold syndrome type 1 (Fig. 6f).