Signal transduction refers to movement of signals from outside the cell to inside the cell as well as the movement of signals within a cell.
Signal transduction can be initiated by the binding of ligands to membrane bound receptors or the entering of small molecules through the cell membrane. In the cellular environment signals are often propagated through phosphorylation events including kinases and phosphatases. Small molecules can also enter directly into the nucleus, effecting the activity of nuclear receptors that constitute a distinct class of transcription factors. The eventual outcome of signal transduction is an alteration in cellular activity and changes in the program of genes expressed within the responding cells.