Who are the 'South Bank seven'? And what might they do next?
In a boiling hot, cramped room in a swish venue on the south bank of the Thames this morning, a small group of MPs made a big statement.
Luciana Berger, Chris Leslie, Chuka Umunna, Mike Gapes, Anne Coffey, Angela Smith and Gavin Shuker all know they face an extremely bumpy future. But they hope together they'll start as a few, and end up being a group for the many.
Their reasons for quitting are both historic and immediate.
The splinter has been a long time coming because for a couple of years these MPs have been part of the large chunk of the Labour parliamentary party which had grave concerns about Jeremy Corbyn's leadership.
Angela Smith, for example, now admits that during the 2017 election she was telling voters in her constituency that her leader could not, and would not be prime minister, despite campaigning for the party he was at the top of.
The seven who have decided to quit are not the only MPs I know of who did that.
There have been fears, many publicly articulated but expressed with more fury in private, about the leadership's attitude to security policy, to Nato, as well as Jeremy Corbyn's response to the Skripal attack and his attitude to the Trident nuclear deterrent.
One of those who has left described it today as "Marxism in disguise", illustrating the deep-seated, long-held and profound differences of world view.
Add to that the hurt and concern inside the party over anti-Semitism that has built up over the last year, what the departing group have described as institutional racism towards Jewish people.
Whatever else, let that sink in for a moment. That in 2019, a group of MPs believe that our main opposition party is institutionally biased against a minority group.
But consider too the Labour leadership's hesitancy in campaigning full throttle for another referendum on staying in the EU, and the group, all of whom believe there should be another referendum, felt they had no choice but to quit.
To do so goes against the grain of our tribal politics. Some of their colleagues are openly furious, accusing them of being "cowards". Others are responding more in sorrow than in anger.
This is not an easy moment for anyone in the Labour party, and you could not have sat in that stuffy room this morning and felt it was an easy moment for any of those leaving either.
Many other Labour MPs and members will see this as nothing less than a betrayal. And in our first-past-the-post system it is very hard to see in the short term, what kind of impact this group will have.
So far they are not a political party, although they say they may evolve into one. So far they have no leader, and no policy programme as such. They are clearly open to welcoming disgruntled members of the Conservative party too.
Their view is that our whole political system is broken and neither the Tories nor Labour are fit for purpose. And it is possible within days that they might be joined by a sprinkling of Tory MPs.
This splintering might, just might - in time - turn into a much bigger redrawing of the landscape.
For now though that is way off. And this is first and foremost about the Labour Party - the seeds of the splinter sown more than three years ago, bearing bitter fruit just when Parliament's biggest decisions over Brexit are about to be made.
MPs still in the party will have a variety of reactions, from fury to sadness.
But few of them now could pretend there isn't a problem, even prompting an astonishing admission from the party's deputy leader, Tom Watson, who - remember - is also elected by the members who so overwhelmingly supported Jeremy Corbyn.
"I love this party. But sometimes I no longer recognise it," he said.
A warning that despite the government's many and multiple problems, it is Labour that's losing members and losing MPs.