Elon Musk and Apple cofounder Steve Wozniak are among the prominent technologists and artificial intelligence researchers who have signed an open letter calling for a six-month moratorium on the development of advanced A.I. systems.

In addition to the Tesla CEO and Apple co-founder, the more than 1,100 signatories of the letter include Emad Mostaque, the founder and CEO of Stability AI, the company that helped create the popular Stable Diffusion text-to-image generation model, and Connor Leahy, the CEO of Conjecture, another A.I. lab. Evan Sharp, a cofounder of Pinterest, and Chris Larson, a cofounder of cryptocurrency company Ripple, have also signed. Deep learning pioneer and Turing Award–winning computer scientist Yoshua Bengio signed too.

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The letter urges technology companies to immediately cease training any A.I. systems that would be “more powerful than GPT-4,” which is the latest large language processing A.I. developed by San Francisco company OpenAI. The letter does not say exactly how the “power” of a model should be defined, but in recent A.I. advances, capability has tended to be correlated to an A.I. model’s size and the number of specialized computer chips needed to train it.

Read more: Fortune