A top nuclear expert has resigned from the State Department following President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE's decision to pull out of the Iran nuclear deal.

Richard Johnson, acting assistant coordinator in the agency’s Office of Iran Nuclear Implementation, stepped down this week, according to Foreign Policy.

Johnson had been involved in negotiations with European countries working to save the deal.

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“I am proud to have played a small part in this work, particularly the extraordinary achievement of implementing the [deal] with Iran, which has clearly been successful in preventing Iran from acquiring a nuclear weapon,” Johnson said in an email to colleagues about his departure.

Trump announced Tuesday that he would pull the U.S. out of the Obama-era nuclear pact, moving to reinstate economic sanctions against Iran.

Officials told Foreign Policy that the 38-year-old Johnson’s departure is the latest symptom of what they have dubbed a “brain drain” in the agency under the Trump administration. About 60 percent of top-ranking career diplomats left the department under Trump.

Johnson will join the Nuclear Threat Initiative, a nonprofit aimed at preventing attacks with weapons of mass destruction.