James Purnell's resignation is clearly a significant development in deepening the Labour leadership crisis.
Matthew d'Ancona, John Rentoul, Phil Collins, and Martin Kettle all believe that it is all over for Gordon Brown.
That could be how it turns out. But it ain't necessarily so.
Potentially, almost as significant are the statements from Cabinet Ministers including David Miliband and Andy Burnham that they will not be resigning. This potentially creates a firewall. If there is no change in the positions of Miliband or Burnham overnight, then there is little question that the Prime Minister could complete a reshuffle.
A single resignation can not enforce the departure of the Prime Minister; that would take a collective refusal to serve. There have been unambiguous public statements for Gordon Brown from John Hutton, Liam Byrne, Tessa Jowell and Caroline Flint, as well as from Alan Johnson, Jack Straw, John Denham and others.
If the Cabinet remain supportive of Brown, then the focus of media attention might shift to the backbenches. But the formal leadership challenge mechanism - nominations for a named candidate; followed by a special conference card vote to decide if a leadership contest proceeds - is in practice inoperable, surely by design.
So the point of backbench pressure - if there were sufficient signatures to put a contest on the agenda - is really to get the Cabinet to act.
And, tonight, the vast majority of the Cabinet are still backing the Prime Minister.
(UPDATE: PoliticsHome has links to the statements made from Cabinet and other ministers).