Severity of damage: silent << missense < nonsense < frameshift.

Transition = Purine to purine (eg, A to G) or pyrimidine to pyrimidine (eg, C to T).Transversion = Purine to pyrimidine (eg, A to T) or pyrimidine to purine (eg, C to G).

Silent: Nucleotide substitution but codes for same (synonymous) amino acid; often base change in 3rd position of codon (tRNA wobble).

Missense: Nucleotide substitution resulting in changed amino acid (called conservative if new amino acid is similar to chemical structure.- Sickle cell disease (substitution of glutamic acid with valine).

Nonsense: Nucleotide substitution resulting in early stop codon (UAA, UAG, UGA).

Frameshift: Deletion of insertion of a number of nucleotides not divisible by 3, resulting in misreading of all nucleotides downstream. Protein may be shorter or longer.- Duchenne muscular dystrophy, Tay-Sachs disease

Splice site: Mutation at a splice site → retained intron in the mRNA → protein with impaired or altered function.- Rare cause of cancers, dementia, epilepsy, some types of β-thalassemia.