Objective. To study clinical and neurophysiological data of early infantile epileptic encephalopathy type 14 caused by KCNT1 mutations. Material and methods. For the period 2017 to 2019, 3 non-relative girls with clinical characteristics of epilepsy of infancy with migrating focal seizures (EIMFS) and mutations in the KCNT1 gene are identified and studied. DNA sequencing was performed using the Hereditary epilepsy panel (Next Generation Sequencing on platform IlluminaNextSeq 500, USA). Dynamical video-EEG monitoring was done with “Encephalan-Video” RM-19/26 (“Medicom MTD”, Russia). Results and Conclusion. De novo KCNT1 mutations are identified and studied in three unrelated Russian girls: M.V., 3 years and 3 month old, T.V., 9 month old and M.A., 5 month old. M.V. has the previously unknown mutation in exon 12 (chr9:138656907C>T) with amino acid substitution Arg356Trp. T.V. has the previously described mutation in chromosome 9: 138651532G>A with amino acid substitution Gly288Ser (OMIM: 608167.0010). M.U. has the previously unknown mutation in exon 15 (chr9:138660712A>G) with amino acid substitution Asp480Gly. M.V. has seizure onset at the age of 4 month with behavioral arrest seizures and tonic versive seizures. T.V. developed seizures at 4,5 month in the manner of behavior arrest and ophthalmo-clonic seizures with hyperemia of face. M.U. has neonatal seizures with bilateral tonic-clonic seizures, cyanosis and further development of status epilepticus of alternating hemiconvulsive seizures. Further all the girls develop polymorphic seizures of multiregional genesis up to migrating status epilepticus with typical electro-clinical pattern of EIMFS. Therefore, KCNT1 is likely to be a major gene causing this rare and severe epileptic syndrome.