Protein quaking isoform HQK-5, also known as QKI, is an RNA-binding protein that regulates pre-mRNA splicing, export of mRNAs from the nucleus, protein translation, and mRNA stability. It is involved in myelinization and oligodendrocyte differentiation, and it may play a role in schizophrenia. Defects or deletions in the QKI are associated with astrocytic tumors and may be involved in the pathogenesis of schizophrenia. Recombinant human QKI protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.