Hundreds of employees at AI lab OpenAI have reportedly signed a letter demanding the resignation of the company's board or they will follow co-founder Sam Altman to his new role leading an AI team at Microsoft. The letter was signed by 505 OpenAI employees, representing the majority of the company's headcount. Some of OpenAI's most senior leaders reportedly signed the letter, including former interim CEO Mira Murati and COO Brad Lightcap. Co-founder and board member Ilya Sutskever also signed the letter, expressing regret over his role in Altman's firing. Microsoft CEO Satya Nadella announced on Sunday that Altman and fellow OpenAI co-founder Greg Brockman would head a new independent AI team at Microsoft.