Anti-US4 antibodies enable researchers to detect and measure the US4 antigen in biological samples. This target is a reported synonym of the RPS9 gene, which encodes ribosomal protein S9. This protein is known to function in protein translation and the regulation of cell proliferation, among other biological roles. The human version of US4 has a canonical amino acid length of 194 residues and a protein mass of 22.6 kilodaltons. It is reported to be localized in the nucleus and cytoplasm of cells and widely expressed in many tissue types. US4 is a member of the Universal ribosomal protein uS4 protein family.