The most common nonfunctional TPMT alleles, as defined by certain SNPs, are:
- rs1800460, known as 460A>G or A154T, potentially encoding the TPMT*3A allele;
- rs1142345, known as 719A>G or Y240C, potentially encoding the TPMT*3C allele;
- rs1800462, known as A80P, encoding the TPMT*2 allele
- rs56161402, known as R215H, encoding the TPMT*8 allele
- rs1800584, encoding the TPMT*4A allele
Although these alleles are all relatively rare, they do vary in frequency between ethnic populations; for example, the most common TPMT variant in African-Americans is Y240C (2-3% of all alleles; but 50% of all variants), whereas the most common variant in Caucasians is A154T (5% of all alleles).
More complete details on effects of variations of this gene on drug metabolism are provided PharmGKB.