Mental Retardation (MR) is a common human disorder that may be one of the clinical signs of a syndrome (as in Down syndrome), or it may be associated with metabolic, mitochondrial or developmental disorders. A large group of MR includes the non-specific forms whose only consistent clinical manifestation is the mental handicap. Family studies have demonstrated a relatively large number of X-linked forms that seem to explain the excess of about 30% in affected males. The reported prevalence of X-linked MR is about 0,9-1,4/1000 males. Different mutations in 30 X-linked genes have been identified until now. We identified mutations in GDI1 and RAB39B genes, responsible for human X-linked MR (XLMR) suggesting that vesicular traffic was one of the pathways important for development of cognitive functions, and the study of the role of GDI1 and RAB39B in cognitive processes is our major interest.