Reading Frames and Open Reading Frames (ORFs), Explained
6 min read · Updated June 8, 2026
If you have a stretch of DNA and want to know what protein it could encode, you need to think in reading frames. This guide explains the six frames, what makes a reading frame 'open', and how to home in on the ORF that matters.
What is a reading frame?
The genetic code is read in non-overlapping triplets called codons. Because you can start reading at the first, second or third base, every strand has three possible reading frames. DNA is double-stranded, so the reverse strand adds three more — giving six reading frames in total for any sequence.
What makes a reading frame 'open'?
An open reading frame (ORF) is a stretch of codons that could be translated into protein without interruption: it begins at a start codon (usually ATG, coding for methionine) and runs to the next in-frame stop codon (TAA, TAG or TGA). 'Open' means there is no stop codon breaking it up in between.
Not every ORF is a real gene, and not every gene's coding sequence is the longest ORF — but finding ORFs is the standard first pass when you want to know what a sequence might encode.
Start and stop codons
- Start: ATG (methionine) is the canonical start codon; some organisms use alternatives like GTG or TTG.
- Stop: TAA, TAG and TGA terminate translation and are not translated into an amino acid.
Finding the right ORF
A practical approach is to translate all six frames, then look for the longest ORF or the one that matches your expectations (for example, a known protein length or a Kozak-like context around the ATG). Setting a minimum length filters out the many short ORFs that occur by chance.
Frequently asked questions
- Why are there six reading frames?
- Three come from the three possible starting positions on the forward strand, and three more from the reverse complement strand — six in total.
- Is the longest ORF always the gene?
- Often, but not always. The longest ORF is a good first guess, but real coding sequences are confirmed with start-codon context, homology and expression evidence.