I took 150 base pairs of DNA out of the worm genome from a region somewhere in front of my "gene of interest", made a four fold repeat of that sequence and placed this repeat in front of the jelly fish gene for "GFP" (green fluorescent protein). Then I made worms transgenic with this DNA and: some transcription factor proteins in this living worm bind this piece of DNA and make the worm really produce that GFP protein from the jelly fish DNA!
But those transcription factor proteins are not present everywhere: Only in a few neurons in the head and in the excretory canal along the body (like the kidney in humans) are they active:
What transcription factors are that, and what function does "my gene of interest" have in these tissues?
This picture has all other light then GFP fluorescence filtered away. If you want to have both normal light and GFP ecitation light mixed, look here:
The worm is not so big, this one is a bit less then 1mm of length. Look at human embryonic kidney cells making the very same jelly fish protein at the same magnification: