GDF2 and Hermansky-Pudlak syndrome: Second, we used ALK1-FC, a ligand trap for BMP9, to neutralize BMP9 in an in vitro HPS model, and we also found KLF6, ALK1, ENG and PCNA were significantly upregulated in PMVECs under stimulated CBDL serum, whereas ALK1-FC significantly inhibited the expression of those proteins in this context (Fig. 6C).