cytosineadenineguanine

Cytosineadenineguanine, abbreviated CAG, is a trinucleotide sequence consisting of cytosine (C), adenine (A), and guanine (G). In DNA, this sequence on the coding strand corresponds to the mRNA codon CAG, which during translation specifies the amino acid glutamine (Gln). The genetic code also includes CAA as another codon for glutamine.

CAG repeats: In certain genes, CAG can occur in long tandem repeats. Normal alleles have a limited number of repeats, while expansions can lead to polyglutamine diseases, a group of neurodegenerative disorders. Huntington's disease is the best known example, caused by an expanded CAG repeat in the HTT gene that leads to a glutamine-rich protein fragment. Other conditions include various spinocerebellar ataxias. The severity and age of onset typically correlate with repeat length.

Relevance and context: CAG is among the codons used in protein-coding genes across organisms. Its presence on the coding strand mirrors the mRNA sequence, and the corresponding tRNA carries an anticodon that pairs with CAG during translation.