BBC News
Published
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Three men have emerged as favourites to replace her at the helm.
BBC News NI looks at the careers to date of Sir Jeffrey Donaldson, Edwin Poots and Gavin Robinson.
Sir Jeffrey Donaldson
Sir Jeffrey Donaldson is the DUP's Westminster leader and is currently the longest-serving MP from Northern Ireland.
Born in Kilkeel, the 58-year-old has said his decision to enter politics was influenced by the IRA murder of his cousin and RUC officer Samuel Donaldson in 1970.
He began work for MP Enoch Powell between 1982 and 1984 after joining the Ulster Unionist Party, later leading election campaigns for him in 1983 and 1986.