Imaging molecular function. (a) Light (baseline) image and (b) the contrast-enhanced fluorescence image of an LX-1 tumor implanted into the mammary fat pad of a nude mouse, as indicated by the white arrow. Significant contrast appears between cancer and background on the fluorescence image. The contrast agent used was a synthetic graft copolymer with Cy5.5-quenched fluorochromes, which was activated in the presence of cathepsin D. (c) Dissected tumor in the mammary pad and (d) histology. Published with permission from Weissleder et al .