XPO1 and neoplasm: Here the FCCS based target occupancy measurements were successfully employed to quantify the effects of selinexor treatment on the amount of XPO1 protein detected in cells pre- and post-treatment, to compare the occupancy of XPO1 by selinexor and KPT-8602 in cell lines with varying sensitivity to SINE compounds, to test the impact of freezing samples on the outcome of the assay, to evaluate PBMCs as a possible patient surrogate sample for the assay, and to test whether occupancy could be measured in tumor lysates from mice dosed with selinexor in vivo to mimic patient biopsy material.