In the largest deal in its 17-year history, Affiliated Managers Group Inc. is buying Pantheon Ventures from Russell Investments, a unit of Northwestern Mutual Life Insurance Co.

AMG, a global asset manager in Boston that owns stakes in a series of boutique investment firms internationally, will pay $775 million in cash with the potential for additional payments over the next five years, contingent on Pantheon's growth. AMG said that when the deal closes in the second quarter, Pantheon's management would own a "meaningful stake" in the company and continue to run the firm. The deal was announced Wednesday.

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