In PNAS this week, a team of US researchers present a study on the conformational trapping of mismatch recognition complex MSH2/MSH3 on repair-resistant DNA loops. MSH2/MSH3 discriminates between repair-competent and repair-resistant loops by "sensing the conformational dynamics of their junctions," the authors write. MSH2/MSH3 dissociates from repair-competent loops, but repair-resistant CAG loops trap nucleotide-bound MSH2/MSH3 and don't allow it to dissociate from DNA.