HOTag6 is a ~30-amino-acid tetramerization domain followed by a (Gly-Ser) linker followed by the sequence for human monoubiquitin (UniProt ID: P0CG47, https://www.uniprot.org/uniprot/P0CG47, amino acids 1-76) . This is a designed protein (no natural variant exists). The HOTag6 sequence is from the Shu group at the University of California, San Francisco (https://doi.org/10.1016/j.molcel.2017.12.008).