Bluebird bio shareholders have dragged their feet in tendering shares as part of the company’s sale process, prompting buyers Carlyle and SK Capital to extend their offer deadline once again.

Bluebird investors now have until the end of the day May 28, U.S. EDT, to tender their shares, the acquirer group said, according to a bluebird securities filing Tuesday. The offer was previously scheduled to expire Monday, May 12.

Although no specific reason was given, the investment firms noted that as of the close of business Monday, they had received about 2.5 million bluebird shares. That means only about 25.6% of the Massachusetts company’s nearly 9.8 million outstanding shares were tendered, far below the 50%-plus-one share threshold needed for the deal to close.

Tuesday’s extension was not Carlyle and SK’s first rodeo. The pair had previously postponed the expiration date of its offer to acquire bluebird three times to reach the most recent May 12 deadline. The transaction secured all required regulatory approvals May 5.

The deal was originally announced Feb. 21. Carlyle and SK are trying to purchase bluebird at $3 per share, plus an additional $6.84 per share in contingent value rights if sales of bluebird’s three commercial gene therapies—Zynteglo, Lyfgenia and Skysona—reach $600 million over any 12-month period by the end of 2027.

That target seems far-fetched given that bluebird only recorded $83.8 million in total 2024 revenue. Without the CVR, the transaction is only worth about $29 million.

While Carlyle and SK may have the patience to keep extending the offer—they can follow this strategy indefinitely as long as they abide by proper disclosure rules—bluebird may not have the luxury of time.

Having realized the number of shares tendered wasn’t enough for the deal to close, bluebird Friday warned investors that “it is important to tender now.” The alternative to the sale would be bankruptcy or liquidation, the company said.

If bluebird were not to be acquired, the company would be at “significant risk of defaulting on its loan agreements with Hercules Capital.” The gene therapy biotech would then be forced to shut down, a scenario in which it would be “extremely unlikely” that stockholders would receive any payback on their investments, bluebird said.

Based on continued cost-saving projects, current sales and cost patterns and a loan from Hercules, bluebird expects its existing cash to be able to sustain operations into the second quarter, according to the company’s annual report. The second quarter started in April and ends next month.

Under the terms of the Hercules deal, a default can be declared if bluebird fails to close the merger transaction by June 20 at the latest. That deadline was originally set for April 25 and can be extended twice, subject to certain conditions.

Some bluebird investors might be withholding their shares in the hopes of the company getting another better buyout offer.

A rival bidder, Ayrmid, did emerge in late March with a nicer $4.50-apiece upfront term. However, after three weeks of engagement, Ayrmid didn’t submit a binding proposal, nor had it obtained necessary financing to bankroll the deal, bluebird said in mid-April. As a result, bluebird’s board unanimously reaffirmed its backing of the Carlyle-SK agreement.