The predictions site Polymarket is talking to investors about raising $400 million in funding at a valuation of about $15 billion including the new money, according to two people familiar with the talks.

The financing would add to the $600 million already invested in the funding round by Intercontinental Exchange, the parent of the New York Stock Exchange, which the exchange announced last month without disclosing the valuation. It’s looking to add additional strategic investors beyond Intercontinental Exchange to the round, which could total $1 billion, said a second person.