The fluorescently quenched probe, 6QC-Cy5 is an opticalcontrast agent that exploits a latent lysosomotropic effect for usein fluorescence-guided tumor surgery.35 Upon cleavage by cathepsins, it accumulates inside lysosomal compartmentsof tumor-associated macrophages that populate the tumor microenvironment(Figure 2a). The gene discussed is CTSS; the disease is neoplasm.