Queuine (/kjuːiːn/) (Q) is a hypermodified nucleobase found in the first (or wobble) position of the anticodon of tRNAs specific for Asn, Asp, His, and Tyr, in most eukaryotes and prokaryotes.

However, queuosine can be produced only by bacteria; higher organisms must obtain queuine from the diet, or salvage it from symbiotic microbes: a process for which dedicated enzymatic machinery exists.

Once salvaged, queuine replaces a guanine base in the anticodon of certain tRNAs, where it appears to play a role in ensuring rapid and accurate recognition of the corresponding mRNAs' codons.

[7] In animal experiments using "germ-free" mice, even the total absence of queuine in the diet is not lethal in the presence of an adequate supply of the dietary amino acid tyrosine.

[8] Because the aromatic amino acid hydroxylase enzymes are the rate-limiting steps in the body's biosynthesis of serotonin and dopamine (and subsequent metabolites including melatonin, norepinephrine, and adrenaline), queuine deficiency is under investigation as a potential cause of human diseases linked to a deficit of these neurotransmitters.