In genetics, the 3' UTR (read as 3 prime untranslated region) is a particular section of messenger RNA (mRNA).

Like all strands of nucleic acid, mRNA is directional. One end is the 5' (five prime ) end: the other is the 3' (three prime) end.

When a ribosome uses messenger RNA's sequence to build a protein (the process is called translation), it starts reading from the 5' end, until it encounters a "start" codon. Amino acids are then added to the protein until the enzyme encounters a "stop" codon (three nucleic acids which tell the enzyme to stop). Anything beyond this stop codon is part of the 3 prime untranslated region (3' UTR).

In a diagram:

       start                stop
       codon                codon

---------|-------------------|---------
5'-UTR      translated RNA     3'-UTR

Many functional elements occur in the 3' UTR:

• Polyadenylation signals (usually AAUAAA, or a slight variant) that mediate the cleavage of the transcript ~30bp downstream of the signal, followed by the addition of several hundred adenine residues (poly-A). The poly-A tail is thought to protect the mRNA from degradation.
• SECIS elements, which can occur in the 3' UTRs of eukaryotic mRNAs, direct the ribosome to translate UGA codons as selenocysteines.
• The histone downstream element is analogous to polyadenylation in function, but has different sequence characteristics, and is used only for histone genes.