This review presents the theoretical, practical and geographical aspects of Rett syndrome and other rare diseases, according to the data of the last VIII International Congress in Russia, and the main publications on Rett syndrome. The issues highlighted by the participants remain relevant and determine the direction of modern studies. The presentations made at the symposium helped to form a global concept of the molecular and cellular mechanisms of Rett syndrome and a number of rare genetic/genomic diseases. The article presents a number of domestic findings in the field of Rett syndrome and other rare diseases. The authors also present information on rare diseases associated with the Rett-like-phenotype or with mutations/variations of the MECP2 gene sequence copies. The authors consider the identified chromosomal (genomic) disorders / diseases in the context of rare diseases. This approach to the Rett syndrome studies analysis is quite new in the world research practice. We hope this review to become valuable not only for specialists in the field of rare genetic diseases, but also for the scientists and clinicians studying Rett syndrome and for physicians (pediatricians, geneticists, neurologists, psychiatrists) meeting these patients in their practice.