Here, we report a case of severe PNS in an 80-year-old man with SCLC following a four-year course of atezolizumab, presenting with simultaneous optic neuropathy, parkinsonism and truncal ataxia, and a complex antibody profile, including anti-Hu, anti-CV2/CRMP5, and anti-SOX1. The gene discussed is SOX1; the disease is small cell lung carcinoma.