细胞外的信号通过蛋白与蛋白和蛋白与配体的作用由细胞表面传送到细胞核。细胞通过特殊蛋白质如激素、细胞因子或其它分子传感器而持续感受外界环境变化并作出反应。这些信号分子通过与细胞受体结合发挥作用，将信息传导到细胞核中，在那里特殊基因由于外界环境变化而被激活。信号从细胞表面传到细胞核是经由"第二信息子"完成的，这种信息子可以是胞内蛋白或小分子如环状AMP、肌醇磷酸盐或钙离子。信号传导系统的一个例子是细胞对血液中葡萄糖水平的反应。通过葡萄糖与细胞受体结合感受到高水平，该信号最终传到细胞核中并激活包括胰岛素基因在内的特定基因,然后胰岛素被合成，并离开细胞，起到降低葡萄糖水平的作用。信号传导对于制药业非常重要，因为很多疾病的异常反应是由于细胞或构成信号传导的蛋白质突变造成的。例子包括涉及指导基因成长和分裂的成长因子和胞内蛋白。任何这些分子的变化都会导致不可控制的细胞生长。超过50%的人类肿瘤的发现是通过二个胞内信号通道识别的，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.