Functional RNA structures that contain knots of the form shown in Figs. S5c or S6 are currently unknown or have not been reported. However, pseudoknots are often observed in functional RNA structures, particularly H-type pseudoknots (Fig. S5a) . Single strand RNA sequences such as messenger RNA with introns can have sequences with lengths that number in the tens of thousands of nucleotides. With such a propensity for a few simple cords to become tangled, and, since the cell can have many thousands of protein and RNA strands present within the cellular environment, this suggests that there is a fair amount of effort made within the cell to prevent or get rid of knots [2] .