A nonsynonymous mutation that occurs at the genomic or transcriptional levels is one that results in an alteration to the amino acid sequence in the protein product. A protein's primary structure refers to its amino acid sequence. A substitution of one amino acid for another can impair protein function and tertiary structure, however its effects may be minimal or tolerated depending on how closely the properties of the amino acids involved in the swap correlate.[1] The premature insertion of a stop codon, a nonsense mutation, can alter the primary structure of a protein.[2] In this case, a truncated protein is produced.