For some time, Jo Johnson has struggled with the unfolding reality of Brexit.
A well-respected and liked member of the government, he has decided that what was promised to people during the referendum campaign is now so different to what is on the table that he has decided to quit the government instead.
He's not the first, nor the best-known minister to resign over Brexit. But to leave at this moment, right when Theresa May is trying to stitch together a final deal, could have a serious impact.
And there are several other ministers of a similar rank, who hold very similar views. Not long ago, one of them said, "for me, it could be an issue of party or country".
Jo Johnson's departure could only take away one crucial vote from the government. But it might, just might, be the first of a group of ministers to go who could shift the balance in a very big way.