If you have observed a ligand-dependent initial fluorescence change, it can occur with either of the following patterns shown below. The fluorescence signal can either decrease with increasing ligand concentrations or the signal can increase as a function of the ligand concentration. Both effects can be induced by a binding event but additional tests have to be performed in order to validate the interaction.

Please refer to the SD-test (covalent labeling) or ECP-test (His-tag labeling) for additional details.