MSSM primarily invests in equity securities of mid- and small-cap companies or other investments with similar economic characteristics. The fund defines these companies as those with market caps not exceeding the highest month-end market cap value of any stock in the Russell 2500 or Russell Mid Cap Index for the previous 12 months. To construct the portfolio, the fund employs a multi-manager strategy, with portions of the fund allocated to professional money managers who are responsible for investing the fund's assets. The fund may allocate up to 10% of its assets in foreign securities not traded on a US exchange or the US over-the-counter market. To generate additional income, the fund may lend portfolio securities. On December 9, 2024, MSSM converted from a mutual fund to an ETF structure, starting with $499 million in assets.