ETH has been buoyed by favorable regulatory developments that seem to indicate increasing chances of spot ether ETFs being approved by the SEC after the regulator asked exchanges to update 19b-4 filings, which propose rule changes. As a result, the ether implied volatility curve, which shows market expectations of future volatility across different strike prices and expirations, flattened as 25-delta risk reversals hit year-to-date highs above 18%, and traders heavily bought $4,000 calls for 24 May and 31 May, Presto Research analysts wrote. A Polymarket contract asking if an ether ETF would be approved by May 31 jumped from 10 cents to 55 cents, representing a 55% chance that approval will take place by then.