The drugmaker Shire wants AbbVie to stick with its roughly $55 billion acquisition bid, and it is reminding its U.S. counterpart of the hefty breakup fee it will receive if things don’t work out.

Shire’s U.S.-traded shares shed about a quarter of their value before markets opened Wednesday and a day after AbbVie Inc. disclosed that it was reconsidering a deal it had fought for weeks to put together.

Shire said Wednesday that its board believes AbbVie should follow through with its plan to acquire Shire and reincorporate on the British island of Jersey, where Shire is incorporated.

AbbVie announced Tuesday that it was having second thoughts after the U.S. government created new limitations on the tax benefits of incorporating overseas. The North Chicago, Illinois, drugmaker said in a statement that it had not called off the deal, but its board needs to weigh “the fundamental financial benefits of the transaction” in light of new tax regulations announced by the U.S. Treasury in September.

The regulations aim to limit a practice known as inversion in which a U.S. company reincorporates overseas or combines with a foreign company. These deals can help lower a company’s U.S. tax bill. Numerous U.S. companies, many focused on health care, have announced inversions in recent months.

Shire said Wednesday that it could receive a fee of about $1.64 billion from AbbVie if the deal falls apart.

Shire had rejected several unsolicited bids from AbbVie before the companies reached a deal in July.

U.S.-traded shares of Shire PLC fell $62.58 to $182.06 about two hours before the market open Wednesday. AbbVie dropped 78 cents to $53.35.