Rop (also known as Rom) is a 63 amino acid protein that exists in solution as a dimer forming an anti-parallel four-helix bundle. It regulates plasmid replication in E. coli by increasing the rate of complex formation between two regulatory RNA molecules know as RNA I and RNA II. Rop binds to the two RNA molecules to form a stabilized protein-nucleic acid complex that increases replication inhibition. We intend to elucidate the correlations between structure, stability, and the function of Rop using X-ray crystallography of re-designed Rop proteins, point mutants, and complexes with RNA.