A hypothetical signalling pathway. The figure shows a hypothetical signaling pathway adapted from , containing two transcription factors, a protein kinase and a protein phosphatase. Three levels are shown: DNA, mRNA and proteins, as well the expected transcription network. At the transcript level, gene 1 affects gene 2 which affects gene 5, but gene 2 does not affect genes 3 or 4. If proteins 3 and/or 4 are rate-limiting, this will be reflected in the dependences shown as dotted arrows.