Indirectly measuring adenylation activity can be just as accurate as direct detection depending on the specific method providing the readout. Recently, the Ackerley group described a colorimetric method that quantifies residual ATP present in the reaction mixture after adenylation. The protein BpsA is a single module NRPS consisting of an A-Ox-T-TE domain organization, and it is known to catalyze the formation of the blue compound indigoidine from two molecules of L-glutamine and two molecules of ATP. By adding BpsA and excess L-Glu to an A domain reaction after incubation, the consumption of ATP is stoichiometrically measured as the inverse to the amount of indigoidine formed. This assay provides a more sensitive color change than related malachite green methods listed below, but it does not allow for continuous measurement.