Internal Ribosome Entry Site

**Semantic type:** Gene or Genome

**Definition:** Internal ribosome entry site (IRES) is a translational cis-acting element that directs ribosomes to the start codon by a cap-independent mechanism. IRESs are usually about 450 nucleotides in length and were first identified in picornavirus RNAs. IRESs can fold into a stable secondary hairpin structure to be incompatible with efficient translation by ribosomal scanning. (NCI)

**Synonyms:** - IRES - Ribosome Entry Site