UPDATED 4/21, 10:35 a.m. ET: Elon Musk has secured the approximately $43 billion he would need to acquire Twitter at the price of $54.20 a share, per the Hollywood Reporter. He is now weighing a tender offer to acquire the company, according to a new security filing. 

UPDATED 4/15, 1:07 p.m. ET: Twitter announced that it will attempt to counter Elon Musk’s hostile takeover bid by exercising the “poison pill” defense, or limited duration shareholder rights plan. ​Twitter’s board voted unanimously in support of the plan, which will be in place until April 14, 2023. It would give existing Twitter shareholders the opportunity to buy shares at a discounted price in an effort to dilute the percentage that Musk owns, which is currently at 9.2 percent. The option would open up if someone tries to purchase 15 percent or more of Twitter stock. 

“The Rights Plan is intended to enable all shareholders to realize the full value of their investment in Twitter,” the company said in a statement. “The Rights Plan will reduce the likelihood that any entity, person or group gains control of Twitter through open market accumulation without paying all shareholders an appropriate control premium or without providing the Board sufficient time to make informed judgments and take actions that are in the best interests of shareholders.”

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Elon Musk has offered to buy Twitter, saying in a letter to the Chairman of the Board that the company “needs to be transformed as a private company.”

The letter, per a report from the Associated Press, was included as part of a securities filing on Wednesday and sees the Tesla CEO proposing a buy at $54.20 per share in cash.

“I invested in Twitter as I believe in its potential to be the platform for free speech around the globe, and I believe free speech is a societal imperative for a functioning democracy,” Musk said in his letter to Bret Taylor, Chairman of the Board. “However, since making my investment I now realize the company will neither thrive nor serve this societal imperative in its current form. Twitter needs to be transformed as a private company.”

Describing his offer as his “best and final,” the SpaceX founder added that he will be reconsidering his shareholder position at Twitter should the proposal be turned down. As previously reported, Musk currently stands as Twitter’s biggest shareholder.

“As a result, I am offering to buy 100% of Twitter for $54.20 per share in cash, a 54% premium over the day before I began investing in Twitter and a 38% premium over the day before my investment was publicly announced,” Musk said Wednesday. “My offer is my best and final offer and if it is not accepted, I would need to reconsider my position as a shareholder. Twitter has extraordinary potential. I will unlock it.”

On Thursday, Musk himself tweeted out a link to the filing page for his offer. Also on Thursday, a source was cited in a Reuters report as stating that Twitter is expected to review Musk’s estimated $43 billion buyout offer “with advice from” Goldman Sachs & Co. and Wilson Sonsini Goodrich & Rosati.

“Twitter, Inc. (NYSE: TWTR) today confirmed it has received an unsolicited, non-binding proposal from Elon Musk to acquire all of the Company’s outstanding common stock for $54.20 per share in cash,” a company rep said in a public statement on Thursday. “The Twitter Board of Directors will carefully review the proposal to determine the course of action that it believes is in the best interest of the Company and all Twitter stockholders.”

Musk and Twitter have been paired in a slew of headlines in recent days, starting with the revelation that the former had purchased a 9.2 percent stake. Shortly after this development, however, Twitter CEO Parag Agrawal confirmed that Musk had decided not to join board. A proposed class action lawsuit has also popped up, with a shareholder arguing that they (and other shareholders) were unfairly affected by Musk’s “delayed disclosure” of his stake.