Day 21 contains 48 protein-coding genes, including FASLG, which encodes the Fas ligand. In your body, cells sometimes agree to die for the better good. White blood cells express the Fas receptor on their surfaces, which listens for the Fas ligand. When the Fas ligand attaches, they self-destruct.
Cells in the eye and testis express FASLG all the time, which tells any white blood cells that come nearby to kill themselves. This sets up areas of immune privilege where inflammation would be very dangerous. Similarly, cancer cells can turn on the FASLG gene to evade the immune system.
FASLG is conserved all the way to fish, meaning it is at least 420 million years old.
Click here to see your FASLG gene on Day 21.