In a two-step process using insertional gene targeting and Cre recombinase-mediated cassette exchange, a targeting vector containing Gdf8 under the control of the rat myosin light chain 1F promoter and enhancer, as well as an SV40 small tumor antigen intron and polyA signal was inserted into the locus. This vector was targeted using homology arms corresponding to nt 31165-39425.