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Figure 6a. Optical imaging of enzyme activity in a human LX1 tumor (2-mm diameter) xenografted into the mammary fat pad of a mouse. (a) Light image. (b) Raw near-infrared image. Note the bright tumor (arrow in b) in the chest. This appearance is due to enzymatic activation of the optically quenched cathepsin imaging probe. (For more information on this type of imaging, see reference 26.)