a method of forming an array of proteins selected from antigens or antibodies ; said method comprising the steps of expressing in a recombinant cell , a fusion protein which comprises either an antigen or an antibody binding protein , fused to a peptide having up to 50 amino acids , which peptide comprises amino acid sequence of seq id no 1 lx 1 x 2 ix 3 x 4 x 5 x 6 kx 7 x 8 x 9 x 10 where x 1 is a naturally occurring amino acid , x 2 is any naturally occurring amino acid other than leucine , valine , isoleucine , tryptophan , phenylalanine or tyrosine , x 3 is phenylalanine or leucine , x 4 is glutamine or asparagine , x 5 is alanine , glycine , serine or threonine , x 6 is glycine or methionine , x 7 is isoleucine , methionine or valine , x 8 is glutamine , leucine , valine , tyrosine or isoleucine , x 9 is tryptophan , tyrosine , valine , phenylalanine , leucine or isoleucine and x 10 is any naturally occurring amino acid other than asparagine or glutamine ; where said peptide is capable of being biotinylated by a biotin ligase at the lysine residue adjacent to x 6 ; biotinylating said peptide of the fusion protein at the lysine residue adjacent x 6 ; isolating the biotinylated fusion protein ; applying the biotinylated fusion protein to an avidin or streptavidin coated non - porous support ; forming an array of at least three different proteins on the support by either where the fusion protein comprises an antigen , carrying out steps to the desired number of times to form an antigen array ; or where the fusion protein comprises an antibody binding protein , applying to said protein , either prior to or after step a plurality of different antibodies or binding fragments thereof .