An ORF or the open reading frame is a continuous stretch of codons in mRNA that contain a start codon (usually AUG) and a stop codon (usually UAA, UAG or UGA).


A reading frame is a stretch of DNA which a ribosome uses as a template to synthesize protein. In most cases, a reading frame is read in a continuous fashion, from the 5′ to 3′ direction. However, some reading frames contain either a start codon (ATG, AUG) or stop codon (TAA, TAG, TGA). These start and stop codons are required to ensure that the correct amino acids are incorporated into a protein. In such cases, the reading frame must be “open” in order to allow the ribosome to bind to the DNA at the start of the sequence, and then to move along the template to read the bases downstream, 5′ to 3′.


An open reading frame (ORF) is the part of a reading frame that has the potential to be translated.


A sequence of nucleotides in a DNA molecule that has the potential to encode a polypeptide or protein. It starts with a start codon triplet (ATG), is followed by a string of triplets each of which encodes an amino acid, and ends with a stop triplet (TAA, TAG or TGA).

This term is used when, after the sequence of a DNA fragment has been determined, the function of the encoded protein is not known.
Several features of the ORF are taken into consideration before analyzing their putative function

• whether it actually encodes an expressed protein,
• its length,
• the presence of a “Kozak” sequence upstream of the ATG (implying a ribosome might actually bind there and initiate protein translation), whether the ORF exists within the coding region of another gene,
• the presence of exon/intron boundary sequences and their splicing signals and
• the presence of upstream sequences that could regulate expression of the putative gene.

The existence of open reading frames is usually inferred from the DNA (rather than the RNA) sequence.