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SCLC is frequently associated with paraneoplastic syndromes, including dermatomyositis. Patients with malignancy-associated dermatomyositis express a specific autoantibody pattern usually positive for anti–transcription intermediary factor 1-γ (TIF1-γ), suggesting anti–TIF1-γ plays a role in development of malignancy-associated dermatomyositis. We present a case of a patient with SCLC, paraneoplastic dermatomyositis, positive anti–TIF1-γ, and a point mutation in TIF1-γ coding gene, with prominent clinical response to chemoradiation. We suggest that this point mutation is pathogenic, providing evidence for the development of paraneoplastic dermatomyositis through immune cross-reactivity.