Introduction

Neurogenetic disorders represent a wide spectrum of diseases defined as “clinical diseases caused by a defect in one or more genes, which affects the differentiation and function of the neuroectoderm and its derivates”: they can be divided into type 1 (when they result from a malfunction of a gene expressed in the neuroectoderm) or type 2 (in which neurological manifestations are caused indirectly by an abnormal gene not expressed in the nervous system);1 most of them can be classified in the group of ‘rare diseases’. Neurology has benefited from the genetic advances over the last three decades, transforming our understanding of nervous system disorders. Although the first clinical descriptions of certain neurogenetic disorders were reported in the nineteenth century, the first gene discoveries date back to the 1980s; since then, many genes have been identified with mutations implicated in an increasing number of neurogenetic disorders. It has been estimated that brain disorders may represent 60–70% of the total genetic disorders in clinical child neurology.2 In 2013, the total number of entries for genetic diseases in the OMIM website was 21 565 (http://www.ncbi.nlm.nih.gov/Omim/mimstats.html). This growth of discovered disease-causing gene mutations leads to difficulties in the classification of genetically heterogeneous hereditary diseases, especially in many neurogenetic disorders.