A codon that signals the end of a gene. When RNA polymerase reaches one of these, it ceases transcription and releases the RNA produced. There are two broad types - codons that almost always function as stop codons (UAA, UAG and UGA - note that these are the RNA sequence, not the DNA one). Other codons will function as stop codons depending on the sequence surrounding them, the presence of various proteins bound to the DNA and the same host of other factors that make genetics a lot messier than it sounds at school.