 Many more gene sequences are known than protein structures . Further , the set of solved structures is biased toward proteins that can be easily subjected to the conditions required in X @-@ ray crystallography , one of the major structure determination methods . In particular , globular proteins are comparatively easy to crystallize in preparation for X @-@ ray crystallography . Membrane proteins , by contrast , are difficult to crystallize and are underrepresented in the PDB . Structural genomics initiatives have attempted to remedy these deficiencies by systematically solving representative structures of major fold classes . Protein structure prediction methods attempt to provide a means of generating a plausible structure for proteins whose structures have not been experimentally determined . 
