Luke Prosser's first-half strike helped Colchester beat MK Dons 1-0 and inflict a first home league defeat of the season on their Sky Bet League Two promotion rivals.
Prosser's first goal of the season arrived after 11 minutes and Paul Tisdale's Dons could not find a way back into the game against a resolute U's defence.
Victory saw third-placed Colchester cut the gap on their second-placed opponents to two points.
The visitors took the lead when defender Prosser reacted sharply to Colchester's first corner, looping a right-footed effort over the head of Dons goalkeeper Lee Nicholls.
Dons substitute Chuks Aneke went close five minutes into the second half, but the frontman's long-range shot flew inches over the crossbar.
The hosts piled on the pressure and Ousseynou Cisse went close with an effort of his own moments later.
Luke Norris should have doubled Colchester's lead with 15 minutes to go, but John McGreal's side held out for an impressive victory.