Congenital myasthenic presynaptic syndrome type 20 - neuromuscular disease with autosomal recessive inheritance caused by homozygous or compound heterozygous mutations in the SLC5A7 gene, characterized by severe hypotonia associated with episodic apnea soon after birth of the child. This article describes the case of SLC5A7 gene mutation detection in girl (age 5 month) with respiratory failure, hypoxic-ischemic encephalopathy, severe muscle hypotonia. The previously described homozygous variant of the nucleotide sequence in the 6 exon of the SLC5A7 gene (p.Ile291Thr) was detected by whole-exome sequencing. This mutation was validated by the Sanger sequencing method in patients. The mother of the child has a mutation in the heterozygous state, the father has no the same mutation. One cannot exclude the presence of gonadal mosaicism and/or «false paternity» in the father in view of the presence of a homozygous mutation in the child and the absence of a second mutation in the parents. The presented clinical observation of a patient with a rare genetic form of myasthenia with respiratory failure shows the importance of DNA diagnostics using the method of whole-exome sequencing in order to search for a molecular defect and to determine the further option of patients management with this severe pathology.