by the above described methods typically said modified membrane spanning protein have one or more improved functional properties relative to the wild type membrane spanning protein e g higher crystallizability relative to wild type in accordance with still another embodiment of the present invention there are provided methods to modulate the crystallizability of a membrane spanning protein said method comprising generating a polynucleotide library wherein a sufficient number of bases of a polynucleotide encoding said membrane spanning protein and or a sufficient number of amino acid residues of said membrane spanning protein are randomly modified so as to increase the crystallizability thereof inserting the modified polynucleotides of said second polynucleotide library into a construct comprising a signal sequence and a first marker sequence wherein said marker sequence is in frame with said modified polynucleotide and encodes one or more genes that overcome the sensitivity of said host to the presence of selective pressure agent s conditions and optionally a second marker sequence wherein said second marker sequence when present is downstream of said signal sequence but upstream of said modified polynucleotide thereby producing modified polynucleotide containing constructs transforming suitable host cells with the modified polynucleotide containing constructs and selecting those cells which survive exposure to said selective pressure agent s conditions e g antibiotic and identifying the modified.