GDF11 and cardiac hypertrophy: Using heterochronic parabiosis, Loffredo and colleagues demonstrated that blood from young mice reverses signs of cardiac hypertrophy in aged animals (Loffredo et al. 2013), an effect seemingly mediated by the circulating protein growth differentiation factor 11 (GDF11), a member of the transforming growth factor-β (TGF-β) superfamily (Loffredo et al. 2013; Poggioli et al. 2016).