The party has an abstentionist policy, with all of its MPs refusing to attend Parliament.
BBC: Sinn Fein's Martin McGuinness resigns as Mid-Ulster MP
2.
The party has four MPs but has a long-standing abstentionist policy.
BBC: Sinn Fein gives evidence to Westminster committee
3.
The SDLP's Patsy McGlone described the other candidates' decisions as "truly bizarre" and quipped that he had heard of abstentionist MPs before but never abstentionist campaigns.
BBC: Mid Ulster by-election: Two withdraw from TV debate