This is due to a thymidine to cytosine mutation at the second codon position. A nonstop mutation is a point mutation that occurs within a stop codon. Sequencing analysis revealed a homozygous single substitution determining a stop codon. The tRNA is also covalently attached to the amino acid specified by the complementary codon. A transcriptional terminator is located 69 bases downstream from the translational termination codon. For instance, mutation biases are frequently invoked in models of codon usage. Only two amino acids are specified by a single codon each. The methionine codon AUG is also the most common start codon. The ribosome assembles on the start codon (AUG), located within the Kozak sequence. Similar observations were made regarding mutations in the position -5 from the start codon, AUG. The A-site binds the incoming tRNA with the complementary codon on the mRNA. There is a different tRNA molecule for each codon. The RBS in prokaryotes is a region upstream of the start codon. It is encoded by the codon UGG. It is encoded by the codon AUG.