In the last decade, anti-neurons autoantibodies have been found in serum and cerebro spinal fluid of patients suffering from neurological paraneoplastic syndrome. This discovery has made possible to improve the knowledge of these syndromes as well as to characterize some proteins specific to the nervous system, unknown until now. Paraneoplastic encephalomyelitis can manifest together with an ataxic sensitive neuropathy, a limbic encephalitis, a brainstem or cerebellum syndrome. This encephalomyelitis is almost always associated with a small cells lung cancer and auto-antibodies, called anti-Hu, which recognize all the neurons of the nervous system. Patients with paraneoplastic cerebellar degeneration present an antibody, called anti-Yo, directed against Purkinje cells. In this case, the tumor is a gynecologic cancer. Patients presenting with an opso-myoclonus and a breast cancer have an antineurons anti-body, called anti-Ri, which is absent when the opso-myoclonus is associated with a lung cancer or a neuroblastoma. These three antibodies are the most frequently found and the best studied, but others, rarer, have been described. The genes coding for the proteins recognized by these three antibodies have been cloned. These proteins seem to play a major role in the neuronal maturation and homeostasis. These antibodies prove to be irreplaceable tools to study the phenomenons subtending the neuronal degeneration and the cellular proliferation.