The three stop codons in the standard genetic code are UAA (ochre), UAG (amber), and UGA (opal/umber). They signal termination of translation and are not recognised by any aminoacyl-tRNA (in the standard code). GUG (Guanine-Uracil-Guanine) is a sense codon that codes for valine (Val). It can also occasionally serve as a start codon in prokaryotes (coding for fMet), but it is not a stop codon.