The stop codon is "UAG," also known as amber. Stop codons are sequences of three nucleotides in RNA that signal the end of protein synthesis. When the ribosome reaches a stop codon during translation, it releases the completed protein and the process is terminated. There are three stop codons in the genetic code: "UAG," "UAA," and "UGA." These are also known as the amber, ochre, and opal codons, respectively. "UCA" is not a stop codon.