part_of part of cell communication signal transduction The cellular process in which a signal is conveyed to trigger a change in the activity or state of a cell. Signal transduction begins with reception of a signal (e.g. a ligand binding to a receptor or receptor activation by a stimulus such as light), or for signal transduction in the absence of ligand, signal-withdrawal or the activity of a constitutively active receptor. Signal transduction ends with regulation of a downstream cellular process, e.g. regulation of transcription or regulation of a metabolic process. Signal transduction covers signaling from receptors located on the surface of the cell and signaling via molecules located within the cell. For signaling between cells, signal transduction is restricted to events at and within the receiving cell. signal transduction by cis-phosphorylation signal transduction by conformational transition signal transduction by protein phosphorylation signal transduction by trans-phosphorylation signaling cascade signaling pathway signalling cascade signalling pathway cellular process signaling regulation of cellular process cellular response to stimulus