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.