The present invention provides methods, compositions, and kits for the treatment of disorders associated with overactivation of FGFR3, such as achondroplasia; bone or cartilage disorders; or vascular smooth muscle disorders, or for the elongation of bone. In some embodiments, the present invention provides polypeptides having a natriuretic peptide fused to an Fc domain of an immunoglobulin. Such polypeptides can be administered to subjects, e.g., subcutaneously, to treat a disorder associated with overactivation of FGFR3, a bone or cartilage disorder, or a vascular smooth muscle disorder, or to elongate bone. The invention also features nucleic acid molecules encoding such polypeptides and the use of the nucleic acid molecules for treating disorders associated with overactivation of FGFR3, bone or cartilage disorders, or vascular smooth muscle disorders, or for elongating bone.