That's a legitimate and important catch — auxins (indole-3-acetic acid and synthetic analogs like NAA, 2,4-D) are genuinely used commercially for fruit ripening and rooting, so "not found in food" fails for auxin specifically. Good that you flagged it before it got locked into the design. This actually points to a broader rule worth applying to *any* candidate switch-2 molecule: **anything derived from or resembling a plant hormone, natural product, or agricultural chemical is disqualified**, because global food supply chains have too many uncontrolled exposure routes. You want something **fully synthetic with no natural biosynthetic pathway anywhere** — man-made from scratch, not found in any organism, plant, or existing pharmaceutical. A better-fitting option: the **destabilizing domain (DD) system with Shield-1**. - Shield-1 is a fully synthetic small molecule — not a hormone, not derived from any natural compound, has no agricultural use, no dietary presence, and isn't used clinically for anything else. - Mechanism: a destabilizing domain tag is fused to an essential protein; without Shield-1 present, the tagged protein is constitutively degraded. Give Shield-1, and the protein stabilizes (this is normally used the *opposite* way — to keep a protein alive only when the drug is present). For a kill switch, you'd invert the logic: fuse the DD tag to something that promotes apoptosis when *stabilized*, so administering Shield-1 stabilizes the pro-death protein and triggers the kill. - Mechanistically this is **protein-stability-based**, not caspase-cascade-based like iCasp9 — genuinely a different failure pathway from switch 1, which is what you want for true redundancy against a single mutation knocking out both. So the revised pair: 1. **iCasp9 + rimiducid** — synthetic dimerizer, triggers apoptotic cascade 2. **DD-tagged pro-apoptotic protein + Shield-1** — synthetic stabilizer, triggers via protein-stability control, no relation to rimiducid's chemistry Both molecules are purpose-built lab/therapeutic tools with no natural production, no agricultural use, and no presence in any standard diagnostic panel — they clear your "resistant to daily exposure and tests" bar cleanly, unlike auxin. Want to move to the metastatic-gene knockout next, or start drafting the actual proposal structure?