Also disclosed are methods of making the cross linkers, as well as methods of using the cross linkers, e.g., to deliver a biologically active substance across the membranes of selected cells in a heterogeneous cell population; once inside the cell the active substance is released, intact, by {HEAD ~ the transient, mild acidity} of {TAIL ~ certain cell structures} 
of
Also disclosed are methods of making the cross linkers , as well as methods of using the cross linkers , e.g. , to deliver a biologically active substance across the membranes of selected cells in a heterogeneous cell population ; once inside the cell the active substance is released , intact , by { HEAD ~ the transient , mild acidity } of { TAIL ~ certain cell structures }
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