- Tamm R, Oselin K, Kallassalu K, Magi R, Anier K, Remm M, et al. Thiopurine S-methyltransferase (TPMT) pharmacogenetics: three new mutations and haplotype analysis in the Estonian population. Clin Chem Lab Med. 2008;46:974-9 pubmed publisher..Our results point to several markers that predict reduced enzyme activity. None of the identified markers were associated with high enzyme activity. ..