Conflict of interest: O. Sitbon reports grants, personal fees and non-financial support from Actelion Pharmaceuticals, Bayer and MSD, grants from GlaxoSmithKline, personal fees from Gossamer Bio, Acceleron Pharmaceuticals and Ferrer, outside the submitted work. Conflict of interest: A. Reis reports grants, personal fees and non-financial support from Actelion Pharmaceuticals, outside the submitted work. Conflict of interest: M. Humbert reports grants and personal fees from Actelion, Bayer, GSK and Acceleron, personal fees from Merck and United Therapeutics, outside the submitted work. Conflict of interest: A. Vonk Noordegraaf is supported by the Netherlands CardioVascular Research Initiative (CVON-2012-08 PHAEDRA, CVON-2017-10 DOLPHIN-GENESIS) and the Netherlands Organization for Scientific Research (NWO-VICI: 918.16.610); and has received fees for lectures from Actelion, Johnson & Johnson, Ferrer and Bayer in the past 3 years, and served as a member of the scientific advisory board of Morphogen-XI. Conflict of interest: M. Delcroix reports grants and personal fees for research, consultancy and lectures from Actelion/J&J, personal fees for research, consultancy and lectures from Bayer, personal fees consultancy and lectures from MSD, personal fees for research from Reata, personal fees for research and consultancy from Bellarophon, personal fees for consultancy from Acceleron, outside the submitted work.