In an interview with Mishal Hussain on the Today programme, the DUP leader Arlene Foster insisted that her party's opposition to any customs or regulatory border in the Irish Sea, as part of a UK deal with the EU, is completely unacceptable to them and would amount to a deal-breaker in their confidence and supply relationship with the Conservative Party. We discuss the use of "blood red line" with Professor Rick Wilford, Alban Maginness and Newton Emerson.