Check out the companies making headlines in midday trading:

Palantir Technologies, Dell Technologies — Palantir and Dell jumped 13% and nearly 5%, respectively, following the announcement after the bell Friday that both stocks would join the S&P 500 before the open on Sept. 23. Palantir is set to take American Airlines’ spot, while Dell is going to replace Etsy.

Summit Therapeutics — The biotech stock surged about 65% after the company announced that its lung cancer drug candidate outperformed Merck’s Keytruda in phase three clinical trial. Merck shares dipped 1.5% following the news.

JetBlue Airways — The airline stock popped 8% after Bank of America analyst Andrew Didora upgraded the firm to neutral from underperform. Didora said JetBlue’s “self-help initiatives,” such as revenue improvement strategies, are beginning to show early signs of impact. The analyst doubled his price target to $6, which would represent a 13% upside from Friday’s close. JetBlue last week hiked its third-quarter revenue estimates.

Boeing — The aircraft maker’s stock moved nearly 4% higher after the company reached a deal with a factory workers union, potentially avoiding an expensive strike. 

MarineMax — Shares popped 4% after a Citi upgrade to buy from neutral. The firm said the boat dealer should benefit from the Federal Reserve cutting interest rates.

Arm Holdings — The stock gained 5% after the Financial Times, citing people familiar with the matter, reported that Apple will use Arm’s next-generation chip design in the iPhone 16, which is set to make its debut on Monday.

United States Steel — Shares added 3% on a JPMorgan upgrade to overweight from neutral. The firm thinks the recent pullback presents a buying opportunity and expects shares could retreat further if the company’s planned sale to Nippon Steel falls through.

Cannabis stocks — Cannabis stocks moved higher after former President Donald Trump said in a Truth Social post that he will support legalizing adult marijuana use in Florida. Shares of Tilray Brands advanced 6%, while the AdvisorShares Pure US Cannabis ETF (MSOS) increased more than 9% and the Amplify Alternative Harvest ETF (MJ) rose more than 6%.

— CNBC’s Alex Harring, Hakyung Kim, Yun Li, Jesse Pound and Michelle Fox contributed reporting.