A number of studies demonstrated that mutations in the СНЕК2 gene can increase the risk of oncologic diseases, including breast cancer and that the mutational distribution depends on the genetic structure of populations. In our study we compared the prevalence of c.1100delC, c.444+1G>A, del5395, p.I157T, and p.R145W СНЕК2 mutations in 977 breast cancer patients (Russians, Tatars, Bashkirs, Ukrainians, and individual representatives of other ethnic groups) and in women without any oncologic pathology (n= 1069) from the Republic of Bashkortostan. We found СНЕК2 del5395 mutation with a frequency of 1.23% (12/977) in breast cancer patients, whereas in the control group it frequency was 0.09% (1/1069) (OR:13.28, CI 95%: 1.72-102.33, p = 0.003). Frequencies of c.1100delC and c.444+1G>A mutations in patients controls were 0.4% (4/977) and 0.09% (1/1069), 0.2% (2/1069), respectively. The p.I157T substitution in СНЕК2 gene was the most widespread variant in two studied cohorts (approximately 5%); however, differences in the frequencies between cases and controls did not reach statistical significance. Truncating mutations were mainly found in women of Slavic origin. All three mutations were found in Russians and Ukrainians. СНЕК2 mutations c.1100delC and c.444+1G>A were not found in Bashkirs and Tatars; however, the CHEK2 del5395 deletion was present in Tatars.