Wall Streets top regulator is calling for a broad-based review of the rules that underpin trading in the U.S. equities market.

Gary Gensler, the chair of the

U.S. Securities and Exchange Commission, said on Wednesday that he asked the agencys staff to examine issues related to stock trading including so-called best execution requirements. He also said that the stock-trade settlement process could be shortened to the same day.

Market trading rules and practices have come under intense scrutiny in the wake of wild swings in meme stocks such as

GameStop Corp. and

AMC Entertainment Holdings Inc. While he didnt mention them by name, Gensler directed some of his most pointed comments to market makers like

Citadel Securities LLC and

Virtu Financial Inc. Those wholesale brokerage firms pay retail brokers a fee to execute clients trade orders.

Brokers profit when investors trade, Gensler said in a speech at the Piper Sandler Global Exchange & FinTech Conference. For those brokers who have these arrangements — and not all do — higher trading volume generates more payment for order flow. What makes the current zero-commission brokerage environment different is that investors do not see their costs as theyre executing trades, so they may perceive them as free.

Virtu declined by as much as 8.4% on Genslers comments.

(Adds Gensler remarks in fourth paragraph.)

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