Uncharacterized protein C1orf21, also known as Proliferation-Inducing Protein 13, is a protein that in humans is encoded by the C1orf21 gene. C1orf21 is an intracellular protein that flows between the nucleus and the cytoplasm in the cell. It has been linked with cell growth and reproduction and there has been strong links with various types of cancers. There are no paralogs for this gene, however, many conserved orthologs have been found in all invertebrates. C1orf21 has low to moderate level of expression in most tissues in humans, however, it has the most expression in the skin, lung and prostate.