When scientists at ArborAssays invented the Direct Cyclic Nucleotide EIA immunoassay more than 20 years ago, they knew the kits would be popular. The direct nature of the format for the assays allows cAMP & cGMP in cells to be measured in one step. They went on to refine the assay, developing both colorimetric and chemiluminescent versions for cAMP and cGMP. But what makes them so popular?