In certain aspects, the present invention provides compositions and methodsfor modulating (promoting or inhibiting) growth of a tissue, such as bone, cartilage,muscle, fat, and/or neuronal tissue. The present invention also provides methodsof screening compounds that modulate activity of an ActRIIB protein and/or anActRIIB ligand. The compositions and methods provided herein are useful in treatingdiseases associated with abnormal activity of an ActRIIB protein and/or an ActRIIBligand.