Molecular state machines using HDR template insertion

A molecular state machine is implemented in a cell by designing the cell to use specific homology directed repair ("HDR") templates for repairing double strand breaks in polynucleotides based on a current "state" of the cell. The state may be established by the presence of a molecule in the cell or by the availability of specific cut sites in the polynucleotides of the cell. Different HDR templates or different nucleases may be available for performing HDR based on the state. When the state is changed, the same signal or event will result in a different HDR template being incorporated into the existing polynucleotides of the cell. Signals that are internal or external to the cell may be used to change the state of the cell. The cell may create a log of molecular events, store binary data, or perform other synthetic biology/molecular computing functions based on state.