The Tokyo Stock Exchange Mothers Index of start-up companies has tumbled 18 percent this week, heading toward its worst weekly loss since 2013. The measure, which had surged as much as 84 percent from its February low, is now down 22 percent from its April peak to enter a so-called bear market. Companies that helped propel the measure to its highest level in nine years, including Sosei Group Corp. and Cyberdyne Inc., have been among the stocks leading the way lower.

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