THE PRESENT INVENTION RELATES TO ANTIBODIES INCLUDING HUMAN ANTIBODIES AND ANTIGEN-BINDING PORTIONS THEREOF THAT BIND TO THE EXTRACELLULAR DOMAN (ECD) OF ACTIVIN RECEPTOR-LIKE KINASE-1 (ALK-1) AND THAT FUNCTION TO ABROGATE THE ALK-1/TGF-BETA-1/SMAD 1 SIGNALING PATHWAY. THE INVENTION ALSO RELATES TO HEAVY AND LIGHT CHAIN IMMUNOGLOBULINS DERIVED FROM HUMAN ANTI-ALK-1 ANTIBODIES AND NUCLEIC ACID MOLECULES ENCODING SUCH IMMUNOGLOBULINS. THE PRESENT INVENTION ALSO RELATES TO METHODS OF MAKIND HUMAN ANTI-ALK-1 ANTIBODIES, COMPOSITIONS COMPRISING THESE ANTIBODIES AND METHODS OF USING THE ANTIBODIES AND COMPOSITIONS. THE INVENTION ALSO RELATES TO TRANSGENIC ANIMALS OR PLANTS COMPRISING NUCLEIC ACID MOLECULES OF THE PRESENT INVENTION. FIG. 1