Defective gene expression has being recognized as the main causative agent or a key factor risk of a number of rare diseases,among which several ones affecting the central nervous system (CNS). It may be due to chromosomal microdeletions, spanning one or a few contiguous genes. Alternatively, the number of alleles may be correct, however these alleles may be poorly expressed, because of anomalous cis-active signals modulating transcription, intrinsic mRNA instability and/or poor translatability.