Description of phage λ using the elements of FSLM. In the FSLM model for phage λ the substance generators highlighted in grey produce substances, which bind to binding sites on the left (the connections have been omitted to improve the readability of the figure). The promoters PL 1, PL 2, PR 1, and PR 2are used to model the behaviour of the λ terminator sites tL 1, tL 2, tR 1, and tR 2. The substance generators connected to them are only active, if N is bound to the respective binding sites. The substance "Struc" represents the structural proteins of the phage particles. The shaded grey boxes indicate the number of different states that the corresponding control functions can have. A simulation of phage λ using this model leads to lysogenic behaviour or lytic behaviour. In the lysogenic mode the initially active genes are inactivated, and the substance concentrations decrease rapidly, only CI is produced. The fluctuations of the CI concentration are due to the negative feedback loop involving the binding site OR 3. In the lytic mode, CI and CII are not produced, but the other substance generators are active. The concentrations of Int, N, and Q increase infinitely because of the lack of a negative feedback control. The inset describes the effect of the stress response of the host cell using elements not yet implemented in the FSLM simulator. For a more detailed description of the model see [2, 91]. Reproduced from .