HBD and systemic sclerosis: Interestingly, PH patients (IPAH and SSc-PAH) shared a large number of distinct genes (118) (Figure 1A) that were significantly up-regulated in these PH groups but not in SSc including a total of seven genes CA2, HBA2, HBD, HBG1, HBG2, HBM, HBQ1 shown to be highly enriched for blood gas transport (DAVID: gas transport: p<7×10−10).