A composition for delivering an agent to cells in vitro or to tissues or organs in vivo. The composition comprises either avidin or an avidin fusion protein bonded to a biotinylated agent to form an avidin-biotin-agent complex. A method is further provided for delivering an agent to cells using the avidin-biotin-agent complex which involves administering the complex to an individual subject. The avidin-biotin-agent complex is used in therapeutic and diagnostic methods.
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