Also disclosed are methods of making the cross linkers, as well as {HEAD ~ methods} of using {TAIL ~ 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 the transient, mild acidity of certain cell structures 
of using
Also disclosed are methods of making the cross linkers , as well as { HEAD ~ methods } of using { TAIL ~ 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 the transient , mild acidity of certain cell structures
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