form cys sh on the other hand when cystine levels are high and cysteine levels are low less of the rhbmp 2 elutes as the covalently bonded dimer and more elutes as a peak which is believed to represent a dissociable dimer in which each of the cys360 residues is bonded to a cysteine molecule i e cysteinylated non covalently bonded dissociable dimer the inventors have largely overcome this problem by providing mammalian cell culture compositions comprising a high concentration of l cystine and a low concentration of l glutamic acid in the present invention a high concentration of l cystine is defined as at least about 0 6 mm cystine and is more preferably at least about 0 9 mm at least about 1 05 mm or at least about 1 4 mm the most preferred concentration of l cystine is about 1 4 mm in the present invention a low concentration of glutamic acid is defined as at or below about 0 5mm and most preferably at or below about 0 2 mm the compositions of the present invention are believed to maintain an appropriate balance between l cystine and l cysteine in the culture to promote formation of the covalently bonded dimer this balance is influenced by the concentration of l cystine supplied in the fresh medium however the cells continually import l cystine from the medium and export l cysteine high levels of glutamic acid are believed to restrict this phenomenon and there fore also influence the balance.