Harnessing entropy within the CTD to transmit a message. (A) Dynamic modulation of the CTD seen as a Shannon-type communication system. A noise source is omitted for the sake of simplicity. (B) Essential elements of a programmable cellular communication system. The existence of entropy (i.e. “uncertainty”) within a system, together with the ability to 1) modulate the entropy in a controlled manner and 2) decode the signals, would be sufficient to define a “programmable” (by natural selection) regulatory system.