Serotonin (5-HT) participates in regulation of emotions, mood, sleep, appetite, and cognitive processes. Serotonin transporter (5-HTT, SERT) modulates serotoninergic neurotransmission through reuptake of 5-HT from the synaptic cleft. A variable number tandem repeat polymorphism found in intron 2 of the 5-HTT gene (Stin2 polymorphism) influences 5-HTT expression and is associated with anxiety, depression, suicidal behavior, obsessive-compulsive disorder. The aim of the present study was to investigate Stin2 allele frequencies in populations of Tundra Nenets and Nganasans inhabiting north and east Siberia. The frequency of the STin2.10 allele was 72.1% in Tundra Nenets and 89.4% in Nganasans. The data obtained are consistent with the uniform geographic gradient of STin2 alleles, with low frequency of the STin2.10 allele in East Asia and its increase towards west.