This review is structured as follows: we begin with an overview about the monogenetic neurological disorders associated with epigenetic changes; next, the current knowledge on HPRT1 and APP genes is provided; then, the epistasis between HPRT1 and APP genes related to the neurobehavioral syndrome in LND is described; and finally, we present the construction of expression vectors to study intermolecular interactions between the hypoxanthine-guanine phosphoribosyltransferase (HGprt) enzyme and APP in LND. The gene discussed is APP; the disease is nervous system disorder.