Fluorescence Polarization-Based Binding Assay: Sensitive and quantitative FP-based binding assays were developed and optimized to determine the binding affinities of small-molecule inhibitors to the recombinant Mcl-1, A1/Bfl-1, Bcl-w, Bcl-2, and Bcl-xL proteins. The concentrations of the proteins used in the competitive binding experiments were 10 nM for Mcl-1, 80 nM for Bcl-xL, and 60 nM for Bcl-2. The fluorescent probes, Flu-BID or FAM-BID were fixed at 2 nM for all assays, which binds with Kd values of 2.3 nM, 14.2 nM and 24.1 nM against Mcl-1, Bcl-2 and Bcl-xL respectively. 5 μL of the tested compound in DMSO and 120 μL of protein/probe complex in the assay buffer (20 mM phosphate pH 7.4, 50 mM NaCl, 1 mM EDTA. 0.05% Pluronic F68)s) were added to 96 well black assay plates, incubated at room temperature for 3 h and the polarization values (mP) were measured at an excitation wavelength at 485 nm and an emission wavelength at 530 nm using the plate reader Synergy H1 Hybrid, BioTek.