RPA-interacting protein isoform d, also known as RPAIN, is a single-stranded-DNA binding protein involved in numerous eukaryotic DNA processes including replication, repair and recombination. RPAIN interacting protein has been identified as an adapter protein that is involved in RPA nuclear import instead of the prototypical importin proteins that normally mediate nuclear import. Recombinant human RPAIN protein, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.