RB1 and hereditary hemorrhagic telangiectasia: Because p-RB1 elevation was also found in cells outside the hyperproliferating vessels of the BMP9/10ib retina (Fig. 3D) and because CDK4/6 inhibitors can also block the proliferation of non-endothelial cells that could contribute to the studied vascular defects, we sought to determine whether EC-specific Cdk6 deficiency is sufficient to alter vascular pathology development in HHT mice.