prostate, ovary, and spinal cord cells. Its activity varies sixfold among individual due to
a genetic polymorphism (C → T) that results in a point mutation, namely Thr115Ile. The latter
allele (Ile115) is quite common in Caucasian and Han Chinese (10% frequency) and encodes
a variant of HNMT with decreased catalytic activity and thermal stability. Several other
polymorphisms in the noncoding region of the HNMT gene have also been identified. HNMT
can be inhibited by several antihistamines, quinacrine, amodiaquine, metoprine, and tacrine.