NOT FOR RELEASE, PUBLICATION OR DISTRIBUTION, DIRECTLY OR INDIRECTLY, IN WHOLE OR IN PART, TO US PERSONS OR INTO OR WITHIN THE UNITED STATES, AUSTRALIA, CANADA, SOUTH AFRICA OR JAPAN, OR ANY MEMBER STATE OF THE EEA, OR ANY OTHER JURISDICTION WHERE, OR TO ANY OTHER PERSON TO WHOM, TO DO SO MIGHT CONSTITUTE A VIOLATION OR BREACH OF ANY APPLICABLE LAW OR REGULATION. PLEASE SEE THE IMPORTANT NOTICE AT THE END OF THIS ANNOUNCEMENT.

THE COMMUNICATION OF THIS ANNOUNCEMENT AND ANY OTHER DOCUMENTS OR MATERIALS RELATING TO THE RETAIL OFFER AS A FINANCIAL PROMOTION IS ONLY BEING MADE TO, AND MAY ONLY BE ACTED UPON BY, THOSE PERSONS IN THE UNITED KINGDOM FALLING WITHIN ARTICLE 43 OF THE FINANCIAL SERVICES AND MARKETS ACT 2000 (FINANCIAL PROMOTION) ORDER 2005, AS AMENDED (WHICH INCLUDES AN EXISTING MEMBER OF 4BASEBIO PLC).  ANY INVESTMENT OR INVESTMENT ACTIVITY TO WHICH THIS ANNOUNCEMENT RELATES IS AVAILABLE ONLY TO SUCH PERSONS AND WILL BE ENGAGED IN ONLY BY SUCH PERSONS. THIS ANNOUNCEMENT IS FOR INFORMATIONAL PURPOSES ONLY, AND DOES NOT CONSTITUTE OR FORM PART OF ANY OFFER OR INVITATION TO SELL OR ISSUE, OR ANY SOLICITATION OF AN OFFER TO PURCHASE OR SUBSCRIBE FOR, ANY SECURITIES OF 4BASEBIO PLC.

CAMBRIDGE, UK - 15 October 2026 - 4basebio PLC (AIM: 4BB), a specialist in synthetic DNA manufacturing for next-generation genomic medicines and advanced therapies, announces a retail offer via BookBuild (the "Retail Offer") of up to 97,087 new ordinary shares ("Ordinary Shares") of €1.00 each in the capital of the Company (the "Retail Offer Shares") at an issue price of 515 pence per Retail Offer Share (the "Issue Price").

In addition to the Retail Offer, earlier today the Company announced that it had conditionally raised gross proceeds of £15 million through a placing of 2,664,080 new Ordinary Shares (the "Placing Shares") and a subscription for 1,025,239 new Ordinary Shares by certain directors and existing shareholders (the "Subscription") including those pertaining to the Subscription & Set-Off Agreement as defined in the separate Placing and Subscription announcement released by the Company on 15 October 2026. The Retail Offer, the Placing and the Subscription together comprise the "Capital Raise", and the Placing Shares, Subscription Shares and Retail Offer Shares together comprise the "New Shares". The Placing, Subscription and Retail Offer are each being undertaken at the Issue Price. The Company is proposing to raise up to an additional £0.5 million (before expenses) through the Retail Offer, which would result in aggregate gross proceeds from the Capital Raise of up to £15.5 million. A separate announcement has been made regarding the results of the Placing and Subscription and their terms.

In recognition of their continued support to the Company, the Board believes that the Retail Offer provides the Company's eligible existing retail shareholders with an opportunity to participate in the Capital Raise at the Issue Price. The Issue Price represents a discount of approximately 8.4 per cent to the closing share price of 562.5 pence per existing Ordinary Share on 14 October 2026, being the last practicable date prior to the announcement of the Capital Raise.

For the avoidance of doubt, the Retail Offer is not part of the Placing or Subscription. Full details of the Capital Raise, including the background to and reasons for the Capital Raise and the proposed use of funds received under the Capital Raise, are set out in the separate Placing and Subscription announcement released by the Company on 15 October 2026.

The Retail Offer is conditional, inter alia, upon the passing of the Resolutions by Shareholders at the General Meeting, completion of the Placing and Subscription, and the Retail Offer Shares to be issued pursuant to the Retail Offer being admitted to trading on the AIM market operated by the London Stock Exchange ("Admission"). The Placing and Subscription are not conditional upon completion of the Retail Offer. Admission of the Retail Offer Shares is expected to take place at 8.00 a.m. on 10 November 2026.

The net proceeds of the Placing and Subscription and any proceeds of the Retail Offer will be used to support the Company's commercial development, complete the fit-out of Q-Arc, enhance R&D activities focused on reducing turnaround times and increasing yields, provide additional working capital and extend the Company's cash runway to H2 2028.

Any changes to the expected timetable set out above will be notified by the Company through a Regulatory Information Service. References to times are to London times unless otherwise stated.

The Company values its retail shareholder base and believes that it is appropriate to provide its retail shareholders in the United Kingdom the opportunity to participate in the Retail Offer. The Company is therefore making the Retail Offer available in the United Kingdom through the financial intermediaries which will be listed, subject to certain access restrictions, on the following website: https://www.bookbuild.live/deals/G7VL2Q/authorised-intermediaries. Cavendish Capital Markets Limited will be acting as retail offer coordinator in relation to this Retail Offer (the "Retail Offer Coordinator").