UBM adopts a structural fold distinct from other ubiquitin-binding domains and binds to the hydrophobic surface of ubiquitin centered at L8. Human pol eta contains two functional UBMs, both contributing to replication foci formation. In contrast, only the second UBM of Saccharomyces cerevisiae Rev1 binds to ubiquitin and is essential for Rev1-dependent cell survival and mutagenesis.