Nasdaq OMX Group’s (NDAQ) U.S. trading chief is leaving the exchange for brokerage firm ConvergEx Group.

Eric Noll, executive vice president of transaction services, was considered the top candidate to eventually replace CEO Robert Greifeld. He first joined Nasdaq in 2009, following a long stint at Susquehanna Financial Group.

Noll’s resignation comes as Nasdaq faces criticism over several high-profile glitches, including the so-called Flash Freeze in August.

FBN’s Charlie Gasparino first reported the news of Noll’s departure. Internal and external candidates are being considered to replace Noll, Gasparino reported.

ConvergEx Group, which is owned by private equity firm GTCR, said Noll accepted an offer to become president of the New York-based company. He will transition to the additional position of CEO at the end of this year.

Shares of Nasdaq OMX were down 31 cents at $39.30 at the closing bell.

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