細胞外的信號通過蛋白與蛋白和蛋白與配體的作用由細胞表面傳送到細胞核。細胞通過特殊蛋白質如激素、細胞因子或其它分子傳感器而持續感受外界環境變化并作出反應。這些信號分子通過與細胞受體結合發揮作用，將信息傳導到細胞核中，在那里特殊基因由于外界環境變化而被激活。信號從細胞表面傳到細胞核是經由"第二信息子"完成的，這種信息子可以是胞內蛋白或小分子如環狀AMP、肌醇磷酸鹽或鈣離子。信號傳導系統的一個例子是細胞對血液中葡萄糖水平的反應。通過葡萄糖與細胞受體結合感受到高水平，該信號最終傳到細胞核中并激活包括胰島素基因在內的特定基因,然后胰島素被合成，并離開細胞，起到降低葡萄糖水平的作用。信號傳導對于制藥業非常重要，因為很多疾病的異常反應是由于細胞或構成信號傳導的蛋白質突變造成的。例子包括涉及指導基因成長和分裂的成長因子和胞內蛋白。任何這些分子的變化都會導致不可控制的細胞生長。超過50%的人類腫瘤的發現是通過二個胞內信號通道識別的，即：JAK/STAT和p53/ras通道。

The transmission of an extracellular signal from the surface of the cell to the nucleus by a relay of protein-protein and protein-ligand interactions. Cells continuously sense and respond to their changing environmental conditions via specialized proteins such as the hormones, cytokines, or other molecular sensors. These signal molecules exhibit their effects by binding to cellular receptors after which their message (or signal) gets “transduced” to the nucleus of the cell, where specific genes are activated in response to the alteration in the environmental conditions. The relaying of the signal from the cell surface to the nucleus is mediated by “second messengers,” which can be intracellular proteins or small molecules such as cyclic AMP, inositol phosphate, or calcium ions. An example of a signal transduction pathway is the cell’s response to glucose levels in the bloodstream. High levels are sensed by the binding of glucose to a cellular receptor, and a signal is transduced which eventually leads to the nucleus to activate certain genes including the insulin gene. Insulin is then synthesized, leaves the cell, and acts to lower glucose levels. Signal transduction is of critical importance to the pharmaceutical industry since many disease states are due to the aberrant responses of cells to external agents, or due to mutations to proteins that form the signal transduction machinery. Examples involve the growth factors and the intracellular proteins involved in signaling the genes for cell growth and division. Changes to any of these molecules can result in uncontrolled cell growth. Over 50% of human cancers have been identified as resulting from just two intracellular signaling pathways: the JAK/STAT and p53/ras pathways.