The 8% non-denaturing gel shown contains variations of the basic PX motif, and illustrates that only 6:5, 7:5 and 8:5 form well behaved molecules. These experiments have been performed with PX molecules and their congeners containing about 5 unit tangles in each helical domain. The contents of each lane is indicated above it. 'E.B.' indicates that the molecule has been treated with ethidium bromide; DB is a dumbbell motif, similar to, but shorter than the two-dumbbell PX molecule in Figure 1, in which both molecules have been sealed covalently. In all cases, smears or multimers indicate that PX molecules are not forming stable molecules.