After infecting a host bacterium, bacteriophage lambda, may lie latent (lysogeny), or reproduce and lyse the host bacterium. Only 1 infected cell in 10 million lyses spontaneously, but the switch to lysis is extremely efficient if the bacterial host becomes endangered. The phage protein CI is responsible for maintaining lysogeny by regulating the transcription of itself and genes responsible for lysis. CI mediates a DNA loop by cooperatively binding two triplets of DNA sites separated by 2.3 kbp. Using the tethered particle motion technique and magnetic tweezers we are probing parameters that underlie the remarkable stability of the lysogenic (looped) state.