A codon is a three-base sequence on messenger RNA that specifies an amino acid or a stop signal.
During translation, a ribosome reads messenger RNA in groups of three bases. Transfer RNA molecules carry matching anticodons and deliver amino acids, allowing the ribosome to build a protein in the encoded order. Because four RNA bases are read in groups of three, there are 64 possible codons.
Sixty-one codons specify amino acids, while three—UAA, UAG and UGA—are stop codons in the standard genetic code. AUG commonly serves as the start codon and also specifies methionine. The genetic code is redundant because several codons can specify the same amino acid.
A codon is not the same as an anticodon: the codon is on messenger RNA, whereas the complementary anticodon is on transfer RNA. A promoter is a DNA region involved in transcription initiation, and an intron is a removed non-coding segment in many eukaryotic transcripts.