Hu (ANNA1) is most commonly found paraneoplastic (defined as autoimmune response to tumour antigens) antibody and recognises a family of proteins with molecular size between 35-40 kDA found in both the central and peripheral neuronal nuclei. Weak staining of the cytoplasm can also be observed.

Clinically, Hu antibody is more common in males and found in association with cerebellar ataxia, encephalomyelitis, limbic encephalitis and, sensory and autonomic neuropathy.

In 80% of the cases Hu antibodies reflect autoimmunity to small cell carcinomas.

Image: Staining of Purkinje cell nuclei more than that of the cytoplasm.