Microsoft stock has hit a record high after hiring sacked OpenAI boss Sam Altman and other key figures from the AI firm. The US tech giant, which is a key investor in OpenAI, is the second most valuable company in the world behind Apple. The firm’s market cap on 21 November reached above $2.8 trillion – up $1 trillion since the start of the year. Microsoft chief executive Satya Nadella announced that Mr Altman, who headed ChatGPT creator OpenAI until he was ousted on Friday, would join Microsoft to lead a new advanced AI research team alongside fellow OpenAI co-founder Greg Brockman “We’re extremely excited to share the news that Sam Altman and Greg Brockman, together with colleagues, will be joining Microsoft to lead a new advanced AI research team,” Mr Nadella wrote on X, formerly Twitter. “We look forward to moving quickly to provide them with the resources needed for their success.” Despite his new role, Mr Altman is reportedly keen on returning to his former company following a revolt among OpenAI employees against the board. More than 700 OpenAI workers signed a letter threatening to quit unless the board resigns and reappoints Mr Altman as chief executive. “The process through which you terminated Sam Altman and removed Greg Brockman from the board has jeopardised all of this work and undermined our mission and company,” the open letter stated. “Your conduct has made it clear you did not have the competence to oversee OpenAI.” OpenAI board member Ilya Sutskever said on Monday that he “deeply regrets” his role in Mr Altman’s sacking, adding that he was now working to reunite the company. Following his exit from OpenAI last week, Mr Altman wrote on X: “I loved my time at OpenAI. It was transformative for me personally, and hopefully the world a little bit. Most of all I loved working with such talented people. Will have more to say about what’s next later.” After rumours emerged that he was looking to rejoin his former company after being hired by Microsoft, Mr Altman wrote: “Satya [Nadella] and my top priority remains to ensure OpenAI continues to thrive. “We are committed to fully providing continuity of operations to our partners and customers. The OpenAI/ Microsoft partnership makes this very doable.” Read More Microsoft chief hints Sam Altman may return as OpenAI staff demand board resignation OpenAI staff ‘threaten to quit over ousting of Sam Altman’ Microsoft’s new AI tool cleans up messy backgrounds in video calls