Although proteins on the microarray can have very different amounts, they do share the same epitopes for the purpose of large-scale protein purification [4, 5] . Therefore, probing with anti-epitope antibodies will provide an estimate of the relative protein amounts in each spot on the array. After the protein amount is determined for one spot at row i and column j, ProCAT divides the raw signal intensities S i,j by the protein amount signals A i,j and uses the quotient as an approximation of the strengths of interactions: