CTSS and neoplasm: The NIR signal from Prosense 680 also extended beyond the YFP-expressing tumor cells, suggesting that Prosense 680 is activated by cathepsins secreted into the extracellular matrix16 and/or that Prosense labels nontumor parenchymal cells, such as tumor-associated macrophages.17 Conversely, normal muscle from the contralateral limb had no YFP expression or significant Prosense 680 activation (Fig. 2d).