A method of preparing a species-specific phosphorylation site peptide array for a target organism comprising: a) selecting a plurality of known non-target organism (NTO) phosphorylation site sequences and cognate known NTO phosphorylation polypeptide sequences from one or more NTO, each of the known NTO phosphorylation site sequences comprising at least 5 residues and less than 30 residues; b) identifying a matching target organism (TO) phosphorylation site sequence and cognate TO phosphorylation polypeptide sequence for one or more of the known NTO phosphorylation site sequences; c) determining the matching TO phosphorylation site sequences that correspond to orthologue polypeptides of the cognate known NTO phosphorylation polypeptide sequences; d) selecting the matching TO phosphorylation site sequences determined to correspond to orthologue polypeptides for inclusion on the array; wherein the matching TO phosphorylation site sequences that correspond to orthologue polypeptides are determined by calculating, for each matching phosphorylation site sequence identified in b), a similarity value between the TO phosphorylation polypeptide sequence corresponding to the TO phosphorylation site sequence and a TO polypeptide sequence matching the cognate known NTO polypeptide sequence.