Specific reactions catalyzed by COMT include:
The Val108/158Met polymorphismEdit
In addition to coding for multiple isoforms, the COMT gene itself is also polymorphic. There is a common single nucleotide polymorphism (SNP) located on codon 108 of the soluble form and codon 158 of the membrane bound form (Val108/158Met). This polymorphism results in the substitution of Valine (Val) for Methionine (Met). Individuals who are homozygous for the Met allele exhibit preserved levels of dopamine compared to individuals who are homozygous for the Val allele. 
Associations have been reported between the Val108/158Met polymorphism and performance on the Wisconsin Card Sorting Test (WCST) in healthy adults.  Individuals who have more copies of the Met allele make fewer preservative errors on the Wisconsin Card Sorting Test  but are more likely to suffer from borderline personality disorder.  9% of Kenyans, 31% of Caucasians and 27% of South-west Asians are homozygous for the Met allele. 
- Figure 11-4 in: Rod Flower; Humphrey P. Rang; Maureen M. Dale; Ritter, James M. (2007). Rang & Dale's pharmacology. Edinburgh: Churchill Livingstone. ISBN 0-443-06911-5.