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The given work is devoted to determining the allele length spectrum of 15 microsatellite markers in seven ethnic groups (Slavs, Avars, Chechens, Tatars, Ossetians, Ingush, and Yakuts) in order to verify the quasimonomorphic length range of markers used in an advanced test being developed for assessing microsatellite instability (MSI) in malignant tumors.  The test under development includes a panel of five standard and ten additional microsatellite markers. The standard recommended test comprises five markers (NR-27, NR-21, NR-24, BAT-25, BAT-26) and involves testing both tumor and healthy patient DNA samples. However, in practice, a healthy tissue DNA sample is not always available. This approach carries a risk of false‑positive interpretation of results due to rare polymorphisms. To eliminate this limitation, ten highly monomorphic markers have been additionally included in the panel: TBC-22, LNX-15, SMC-17, MTM-16, PTP-16, FAM-17, CAT-25, HT-17, MAP-17, and ABC-16. As a result, the allele length spectrum was obtained. The quasimonomorphic length range of these markers was confirmed. No sample showed polymorphisms simultaneously in three or more markers out of 15. Thus, the risk of a false‑positive result when assessing MSI using the advanced test under development, without comparison of tumor and normal DNA, is virtually eliminated.