Nature repeatedly repurposes, in that molecules that serve as metabolites, energy depots, or polymer subunits are at the same time used to deliver signals within and between cells. The preeminent example of this repurposing is ATP, which functions as a building block for nucleic acids, an energy source for enzymatic reactions, a phosphate donor to regulate intracellular signaling, and a neurotransmitter to control the activity of neurons. A series of recent studies now consolidates the view that phosphatidylserine (PtdSer), a common phospholipid constituent of membrane bilayers, is similarly repurposed for use as a signal between cells and that the ligands and receptors of the Tyro3/Axl/Mer (TAM) family of receptor tyrosine kinases (RTKs) are prominent transducers of this signal.

大自然反复改变用途，因为同时充当代谢物，能量仓库或聚合物亚基的分子被用来在细胞内和细胞间传递信号。此用途的主要示例是ATP，它充当核酸的基本组成部分，酶促反应的能源，调节细胞内信号传导的磷酸盐供体和控制神经元活动的神经递质。现在，一系列最新研究巩固了以下观点：磷脂酰丝氨酸（PtdSer）是膜双层的一种常见磷脂成分，同样被重新用作细胞之间的信号，以及Tyro3 / Axl / Mer（TAM）家族的配体和受体受体酪氨酸激酶（RTK）的表达是该信号的重要转导者。