Core Insights - S&P Global announced the quarterly adjustment of the S&P 500 index, with AppLovin (APP.US) and Robinhood (HOOD.US) officially joining the index, leading to a post-announcement stock price surge of approximately 7% for both companies [1] - The adjustments will take effect before the market opens on September 22, with AppLovin replacing MarketAxess Holdings (MKTX.US) and Robinhood replacing Caesars Entertainment (CZR.US) [1] - Inclusion in the S&P 500 index typically results in stock price increases due to index funds and passive investment portfolios needing to purchase the relevant stocks [1] - Earlier this year, Datadog (DDOG.US) and DoorDash (DASH.US) were also added to the S&P 500, indicating the growing weight of large tech and digital economy companies in the U.S. stock market [1] Company Specifics - AppLovin is a software company providing targeted advertising for mobile applications and games, showing strong performance since its NASDAQ listing in 2021 [1] - AppLovin previously attempted to acquire TikTok's U.S. operations, which garnered significant attention [1] - Robinhood, also listed on NASDAQ in 2021, gained popularity among retail investors due to "MEME stock" trading [2] - Robinhood has made multiple attempts to join the S&P 500 index but failed in December 2024 and June 2025 [2] Market Reactions - The announcement of the index adjustments typically leads to stock price increases, as seen in past adjustments [1] - The decision not to include Bitcoin treasury company Strategy in the index disappointed some market observers, resulting in a nearly 3% drop in its stock price post-announcement [2]