CCND1 and pulmonary arterial hypertension: While we did not observe a change in the mRNA expression of the proliferation marker Cyclin D1 (encoded by CCND1 gene; Figure 2G) upon treatment of 3D-PAH media tissues with PDGF-BB, an increase in the proportion of nuclei positive for the proliferation marker Ki67 was observed (Figures 2H,I).