IntroductionWhether motor neuron diseases (MNDs) can be considered in some cases of paraneoplastic syndromes is controversial. We report a case of rapidly progressive motor neuronopathy following a diagnosis of breast carcinoma, with a presence of anti-Ri antibodies, and a novel SOD1 gene mutation. ObservationAn 80-year-old woman with mucinous adenocarcinoma of the left breast for 4years developed sub-acute quadriparesis. Myography revealed chronic denervation signs. The patient had serum anti-Ri onconeural antibodies. The diagnosis of paraneoplastic MND was established. Because of a familial history of ALS, a genetic analysis for familial ALS was performed. We identified a novel heterozygous mutation in SOD1 gene, SOD I18del. This mutation may reflect a genetic predisposition to develop a MND, inducing fragility of motor neurons. Neurological improvement was observed after three months of both intravenous gamma globulin and corticosteroids. ConclusionThe present observation supports the idea that MND can be considered as a paraneoplastic syndrome. A combination of anti-Ri onconeural antibodies and a particular SOD1 gene mutation, consisting in risk factor, might be in cause in the process of motor neuron death. When in doubt, paraneoplastic cause should be suspected in the differential diagnosis of MND. Immunotherapy treatment may lead to a favorable outcome.