Computer- generated false- color image of a Drosophila leg imaginal disc. The disc was la- 
beled to visualize the mRNA transcript of the Distal-less gene. A digitized image of a labeled 
disc was prepared and processed to generate an intensity "map" so that different colors 
represent regions of different staining intensity in the original sample. The black region 
(rimmed in purple ) represents the highest RNA concentration. RNA decreases across the disc 
through several color steps (blue, green, yellow, red, pink ), which indicate progressively 
lower concentrations. The point of this manipulation is to demonstrate that a gradient of the 
Distal-less mRNA exists in the disc. The use of false-color intensity mapping highlights the 
idea that different spatial domains could in principle be organized or specified by gradients 
of gene product intensity. 
Research of Stephen M. Cohen. 
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