Email from Brian Wong, M.D., Ph.D., President and Chief Executive Officer of the Company, to employees, dated January 20, 2026 (Form SC14D9C)

Moments ago, we and GSK, a global pharmaceutical company with deep development capabilities in immunology and inflammation, and an extensive commercial footprint in allergy, announced that we have entered into a definitive agreement to be acquired by GSK for approximately $2.2 billion.

Gaining the attention of GSK is a testament to what we've built as an organization. After careful thought, deliberation and consideration, the board and management determined that this is an attractive path forward for our programs, particularly the opportunity we envision for ozureprubart in food allergy, which will benefit from the resources, infrastructure, and capabilities that a global organization like GSK offers. Ultimately, we believe this strategic agreement brings added value to our pipeline, patients, and stockholders.

What happens now?

The acquisition is expected to be completed in the first quarter of 2026, subject to regulatory review and other customary closing conditions.

Until the acquisition closes, it is business as usual. It is important to continue executing on our objectives to advance our programs. During this time, your role, manager, compensation, and benefits remain the same.

What happens next?

At this point, we do not yet have clarity on the future organizational structure. The companies will work together on integration planning and many of you may be involved in those discussions. We are committed to communicating with you transparently as soon as information becomes available and decisions are made.

All Hands Meeting Today

I'm sure you have a number of questions related to this announcement. To that end, we will host an All Staff meeting Tuesday, January 20^th at 9:00am PT. In addition, the Senior Management Team will make ourselves available for one-on-one meetings.

Thank you for your commitment and dedication to RAPT's mission and vision. I'm incredibly proud of what we've accomplished and the resilience our team has demonstrated over the years. We've been driven by our science and commitment to patients - attributes that have led to the opportunity announced today.

With gratitude and my best regards,

Brian

Important Information about the Tender Offer and Where to Find It

The tender offer (the "Offer") for the outstanding common stock of RAPT Therapeutics referred to in this communication has not yet commenced. The description contained in this communication is neither an offer to purchase nor a solicitation of an offer to sell any securities, nor is it a substitute for the tender offer materials that GSK and its acquisition subsidiary will file with the SEC. The solicitation and offer to purchase RAPT Therapeutics' common stock will only be made pursuant to an offer to purchase and related tender offer materials. At the time the Offer is commenced, GSK will file a tender offer statement on Schedule TO and thereafter RAPT Therapeutics will file a solicitation/recommendation statement on Schedule 14D-9 with the SEC with respect to the Offer.

THE TENDER OFFER MATERIALS (INCLUDING AN OFFER TO PURCHASE, A RELATED LETTER OF TRANSMITTAL AND CERTAIN OTHER OFFER DOCUMENTS) AND THE SOLICITATION/RECOMMENDATION STATEMENT ON SCHEDULE 14D-9, AS THEY MAY BE AMENDED OR SUPPLEMENTED FROM TIME TO TIME, WILL CONTAIN IMPORTANT INFORMATION. ANY HOLDERS OF RAPT THERAPEUTICS SHARES ARE URGED TO READ THESE DOCUMENTS CAREFULLY WHEN THEY BECOME AVAILABLE BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION THAT HOLDERS SHOULD CONSIDER BEFORE MAKING ANY DECISION REGARDING TENDERING THEIR SHARES IN THE OFFER.

The offer to purchase, the letter of transmittal, the solicitation/recommendation statement and related offer documents will be made available for free at the SEC's website at www.sec.gov. Copies of those offer documents and all other documents filed by GSK and RAPT Therapeutics will be made available at no charge by directing a request to the information agent for the Offer, which will be named in the Schedule TO to be filed with the SEC. Copies of the solicitation/recommendation statement on Schedule 14D-9 to be filed with the SEC by RAPT Therapeutics will be available free of charge on RAPT Therapeutics' investor relations website at www.investors.rapt.com.

In addition, RAPT Therapeutics files annual, quarterly and current reports and other information with the SEC, which are also made available free of charge on RAPT Therapeutics' investor relations website at www.investors.rapt.com and at the SEC's website at www.sec.gov.