Time required to simulate 1 s in the cell for 1000 molecules. The time required to simulate 1 s in the cell was measured on a Mac Pro 2 × 2.26GHz quad-core Intel Xeon with 32GB memory running Mac OSX 10.6.8. We produced 60 independent simulations for each set of parameters. In addition, each simulation was run for 20 s and only lacI molecules were added to the system. The number of lacI molecules and the association rate was varied according to the system size.