Company: GNMSF
Filing Date: 2025-12-12
Form Type: SC TO-T/A
Source: 0001193125-25-316835
Chunk: 4

Company: GENMAB A/S
Filing Date: 2025-12-12
Form: SC TO-T/A
Chunk 4
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 dividend withholding tax purposes of the New TopCo Shares A immediately prior to the Back-EndCancellation becoming effective, unless an exemption or reduction of Dutch dividend withholding tax is applicable to any particular holder. Unless any Merus Shareholder who receives New TopCo Shares A as a result of the Back-EndMerger demonstrates to New TopCo’s sole satisfaction that such shareholder is entitled to receive its Back-EndCancellation Consideration free of Dutch dividend withholding tax and New TopCo is able to exclude such holder from the tax withholding process, New TopCo will deduct and withhold from the Back-EndCancellation Consideration payable to each such holder such amount of Dutch dividend withholding tax as it is required to deduct and withhold with respect to the making of such payment under Dutch tax law and will remit the amount so deducted and withheld to the Dutch Tax Authority. Whether or not you are ultimately liable for such tax or entitled to other relief will depend on your personal circumstances, and, accordingly, if such tax is not ultimately applicable to you or if you are entitled to other relief, you may be able to recover such amounts or claim other relief therefrom. You should consult your own tax advisor to determine the tax consequences (including the application and effect of any Dutch dividend withholding taxes) to you of the Subsequent Offering Period and the Back-EndTransactions in light of your particular circumstances. The full text of the company announcement issued by Genmab on December 12, 2025, announcing the expiration of the Offer is attached hereto as Exhibit (a)(5)(F) and is incorporated herein by reference. Item 12. Exhibits. Item 12 ( Exhibits) of the Schedule TO is hereby amended and supplemented by adding the following exhibit:

| Exhibit No. |     | Description                                                                                                                                                                                                          |
| (a)(5)(F)   |     | Company Announcement, dated as of December                                                                                                                                                                           
 12, 2025: Genmab Announces Completion of Tender Offer for Outstanding Common Shares of Merus N.V. and Commencement of Subsequent Offering Period (incorporated by reference to Exhibit 99.1 of the Form 6-K as filed 
 by Genmab with the SEC on December 12, 2025).                                                                                                                                                                        |

SIGNATURES After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct. Date: December 12, 2025

| GENMAB HOLDING II B.V. |