Monomers representing DNA tiles: Each DNA tile can have four connections to other tiles. They are made of single stranded sticky ends in Rothemund's experiment using hybridization of real DNA and are represented as binding sites in our experiment with SRSim. Two binding sites d are oriented downwards, two other components u upwards (a). Green arrows set out upwards from a '0' tile, whereas red arrows leave '1' molecules (b). To connect two tiles, a component u from a lower tile can bind to a component d from an upper tile if they are in the same color. The attachment of a new tile (c) happens in two steps. When a '1' tile A has bound to a '0' tile B, it can bind to a '1' tile later. But if the other tile C represents a '0' tile, no second connection stabilizes the bond between A and B, leading to a soon dissociation of tile A.