Proteins consisting of, from amino to carboxyl terminus, a first PDGF-D growth factor domain polypeptide, a linker polypeptide, and a second PDGF-D growth factor domain polypeptide, and materials and methods for making the proteins are disclosed. Each of the first and second PDGF-D growth factor domain polypeptides consists of a sequence of amino acid residues as shown in SEQ ID NO: 2 or SEQ ID NO: 4 from amino acid x to amino acid y, wherein x is an integer from 246 to 258, inclusive, and y is an integer from 365-370, inclusive. The linker polypeptide consists of from 11-40 amino acid residues. The proteins can be used to stimulate the production of bone and/or connective tissue in both humans and non-human animals.