The classic form of Rett syndrome is a rare genetic disorder that most often occurs in girls and affects psychomotor development. It is associated with changes in the MECP2 gene and is one of the most common monogenic causes of developmental delay in girls. In the development of a child, there is usually a short period of "stopping" at about 18 months. The main manifestations of Rett syndrome include: difficulties with speech and movement, gait disorders, repetitive hand movements, and decreased intellectual skills. In addition, the child may experience seizures, sleep and breathing problems, tremors, spasticity, and behavioral patterns similar to autism spectrum disorders. In addition to the classical form, atypical variants of Rett syndrome are possible, which may be more severe or, conversely, milder. In boys, mutations in the MECP2 gene cause severe brain damage from birth. Most of the changes in this gene appear for the first time in a child and are not transmitted from parents. In rare cases, the mutation can be transmitted from a carrier mother, who may not have pronounced symptoms. The absence of changes in MECP2 does not exclude Rett syndrome, as the disease is sometimes associated with other regions of the genome.