Co-option is when during evolution, a gene or protein designed for one function gets used for a completely different function. Give an example. Draw a circle. Label the stages of the cell cycle, including the two major checkpoints. Circle those stages that are transcriptionally active.
Embryonic endoderm lines two tubes within the body.
The Drosophila homolog of this molecule also mediates Wingless signaling. One of the genes you have isolated has no similarity to any gene with known biochemical function, and you call this gene Nu3. You generate antibodies for the protein encoded by Nu3. You use this antibody to stain the developing mouse limb and determine where Nu3 is expressed. Having just completed Zoology 533, you immediately postulate that spacing is controlled by lateral inhibition and you decide to test this hypothesis.