Signal transduction refers to the transmission of a molecular signal in the form of a chemical modification by recruitment of protein complexes along a signaling pathway that ultimately triggers a biochemical event in the cell. Signal transduction occurs when an extracellular signaling molecule activates a specific receptor located on the cell surface or inside the cell. In turn, this receptor triggers a biochemical chain of events inside the cell – known as a signaling cascade – that eventually elicits a response. Depending on the cell, the response may alter the cell's metabolism, shape, gene expression, or ability to divide. The signal can be amplified at any step; thus, one signaling molecule can generate a response involving hundreds to millions of molecules.