Tetracycline-free FBS refers to fetal bovine serum that is certified to be free of tetracycline and related antibiotics. This type of serum is crucial for use in tetracycline-inducible gene expression systems (such as Tet-On and Tet-Off), where even trace amounts of tetracycline can interfere with gene regulation by causing unintended or leaky expression.

Using tetracycline-free FBS ensures tight control over inducible promoters, leading to more accurate and reproducible experimental results. It is especially important in mammalian cell culture experiments where precise timing and dosage of gene activation are required. While slightly more expensive than regular FBS, its reliability makes it a preferred choice for studies involving regulated gene expression and gene function analysis.