The main British parties have generally not stood in seats in Northern Ireland. The by-election exceptions are the 1990 Upper Bann by-election (NI Conservatives and continuing SDP) and the 1995 North Down by-election (NI Conservatives). Prior to the 1970s the Ulster Unionists were effectively the local Conservatives, whilst the Liberals contested some but not all seats. The SDLP has traditionally seen itself as a "sister party" to the British Labour party, and its MPs usually accept the Labour whip in Parliament.