In another codon, the sequence GUA becomes GUU, and the amino acid called aspartic acid changes to valine in the protein. These two amino acids have very different chemical properties. The substitution of one for the other may, or may not, severely alter how the protein folds and functions, depending on where the change occurs in the protein. When the change in the protein does lead to disease symptoms in the patient, the SNP is harmful and is called a mutation.

An example of a SNP causing disease is found in sickle cell anemia. The change in one nucleotide base in the coding region for the hemoglobin beta gene causes the amino acid glutamic acid to be replaced by valine. As a result, the hemoglobin molecule can no longer carry oxygen as efficiently because the structure of the protein is changed dramatically.