What is Tet-Free FBS?

Why standard FBS interferes with Tet systems

Tetracyclines in standard FBS originate from cattle feed additives and veterinary antibiotics. Trace amounts (1–100 ng/mL) remain after processing. In a Tet-On system where doxycycline at 10–1,000 ng/mL drives transgene expression, even 10 ng/mL background from FBS creates partial activation — making the uninduced control unreliable with high background/induction ratio.

Diagnosing Tet system leakiness from FBS

If your Tet-On cell line shows expression in the uninduced state, test by replacing your current FBS with Tet-Free FBS. If background expression drops, FBS tetracycline contamination was the cause — one of the most common findings in inducible expression system troubleshooting.

Key Facts

• Standard FBS may contain trace tetracyclines from cattle feed — typically 1–100 ng/mL
• Tet-On systems are induced by doxycycline at 10–1,000 ng/mL — trace FBS tetracyclines cause significant background activation
• Tet-Off systems are suppressed by tetracycline — trace FBS tetracyclines partially suppress expression even without added inducer
• Tet-Free FBS tested by HPLC or bioassay — confirmed below detection limit for tetracycline and doxycycline
• Used at same concentration as standard FBS (10%) — direct replacement, no adaptation required for most cell lines
• Also required for: CRISPR screening with doxycycline-inducible guides, shRNA knockdown systems, synthetic biology tet-responsive circuits

Further Reading

• What is FBS?
• FBS Lot Testing Protocol
• FBS Portfolio — All Grades

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