This invention relates to crystals of the human Gremlin-1 protein, and the human Gremlin-1 protein in complex with an inhibitory antibody. The invention also relates to the structure of human Gremlin-1 (on its own, or in complex with the antibody) and uses of these structures in screening for agents which modulate Gremlin-1 activity. The invention further provides antibodies which bind an allosteric inhibitory site on Gremlin-1, together with pharmaceutical compositions and medical uses of such antibodies and agents identified by the screening methods.