Along with genetic factors, epigenetic factors have been convincingly demonstrated to determine the development of pathological processes to date. These include hereditary and nonhereditary changes in expression of a specific gene without any corresponding structural changes in its nucleotide sequence. Epigenetic mechanisms regulating gene expression include cytosine methylation in CpG dinucleotides, post- translational modification of histones, ATP-dependent chromatin remodeling, and inactivation of gene expression by various types of noncoding RNAs. At the same time, pathological variants of genes involved in epigenetic mechanisms can lead to both Mendelian syndromes and multi-symptom complexes, in which the main manifestations are mental retardation, delayed physical development, neurological symptoms, and mild signs of dysgenesis. New pathological conditions due to variants in epigenetic machinery genes have been described in the modern literature, and previously described syndromic conditions have been combined into certain "continuous sequences of clinical features" in some cases. The review focuses on germline disorders of genes involved in methylation, demethylation, and methylated DNA binding machinery and their phenotypic manifestations. Somatic variants, which often lead to oncological diseases, are beyond the scope of this review.