Changes in the oligomeric status of MutS protein was probed in solution by dynamic light scattering (DLS), and corroborated by sedimentation ana-lyses. In the absence of any nucleotide cofactor, free MutS protein [hydro-dynamic radius (Rh) of 10–12 nm] shows a small increment in size (Rh 14 nm) following the addition of homoduplex DNA (121 bp), whereas the same increases to about 18–20 nm with heteroduplex DNA containing a mismatch.