Nova-1, a protein expressed in tumors and neurons, is a target antigen in a human paraneoplastic motor disorder [paraneoplastic opsoclonus- myoclonus ataxia (POMA)]. We evaluated the relationship between the function of Nova-1 and its role as a disease antigen. We show that Nova- 1 is a neuron-specific RNA-binding protein with sequence and functional similarities to FMR-1. Nova-1 mRNA is restricted to the subcortical nervous system, and the protein binds to RNA with high affinity. Nova-1 KH domains mediate this RNA binding, and point mutations within them abrogate binding. POMA disease antisera (6/6) recognize the third KH domain but not an inactive point mutant, and affinity-purified antibody blocks Nova-1 RNA binding. Thus, a cardinal feature of POMA is the production of antibodies that inhibit Nova-1-RNA interactions, suggesting such inhibition may cause the neurological disease.