POP4 is one of the protein subunits of the small nucleolar ribonucleoprotein complexes: the endoribonuclease for mitochondrial RNA processing complex and the ribonuclease P complex. POP4 is localized to the nucleus and associates directly with the RNA component of these complexes. This protein is involved in processing of precursor RNAs. Recombinant human POP4 protein, fused to His-tag at N-terminus, was expressed in E.coli.