England captain Dylan Hartley was sent off six minutes after coming on as Northampton Saints crashed to a 37-10 European Champions Cup loss to Leinster on Friday.
Hartley saw red in the 57th minute at Franklin's Gardens for swinging an arm into the head of Sean O'Brien and is now likely to face the familiar threat of disciplinary action.
The result allows Leinster, led by former England coach Stuart Lancaster, to move six points clear at the top of Group Four, leaving Northampton bottom and all but eliminating them from the competition.
Leinster made a lightning start when Rob Kearney broke and fed Garry Ringrose to score his first European try under the posts in the first minute, Isa Nacewa converting.
Nacewa extended the advantage by sending over a penalty in the 10th minute, Saints having been penalised for wheeling under pressure at a scrum.
Northampton got on the scoreboard with a penalty of their own nine minutes later, Stephen Myler delivering with the boot when Leinster were penalised for holding onto the ball.
The hosts began the second half in confident fashion, Ahsee Tuala crossing in the corner four minutes in after quick ball from a penalty in the centre, Myler converting to level the scores.
Nacewa restored Leinster's lead when Tom Wood was spotted playing the ball on the floor and O'Brien then powered his way over on 55 minutes, Nacewa doing the business again to put Leinster firmly back in charge.
Things got worse for Saints two minutes later when Hartley was quickly given his marching orders.
Rory O'Loughlin collected a high cross-field kick from fellow replacement Ross Byrne to score in the corner in the 67th minute, with Jamison Gibson-Park and Nacewa both crossing late to pile more misery on Saints, compounded by a yellow card show to George Pisi with three minutes to go.