Donald Trump’s choice to lead the Securities and Exchange Commission could find it difficult to undo Dodd-Frank Act rules, SEC Division of Economic and Risk Analysis Director Mark Flannery warned Tuesday.

The president-elect has vowed to repeal the law, but Dodd-Frank is extremely complicated, cautioned Flannery, who also serves as SEC chief economist.

The DERA head said Trump’s replacement for outgoing SEC Chair Mary Jo White will have to decide what Dodd-Frank rules can be undone, in what order they should be undone and the implications and unintended consequences of doing away with them.

White is vacating her post when President Obama leaves office January 20.

In an impromptu press conference, she said she has no advice on what major challenges her successor should chose to emphasize.

However, at Tuesday’s SEC Equity Structure Advisory Committee meeting, White said it is important that the continuity of all the agency’s functions need to be ensured, including protecting investors, promoting fair and orderly markets and facilitating capital formation.

She predicted the make-up of securities market will continue to be a major focus of the commission.

When asked what should be the priorities of the new SEC Chair. Republican SEC Commissioner Michael Piwowar declined to comment, but said he would reveal his thinking in due time.