London Welsh are still looking for an Aviva Premiership victory this season after Harlequins came from behind to win 24-13 at the Kassam Stadium.
Marland Yarde's try moments before the break proved to be the turning point of a match which the Exiles had led after opening the scoring on 20 minutes.
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Will Robinson converted after Paul Rowley had crossed the line and then replied to Karl Dickson's try for Quins to help the Dragons into a 10-5 advantage.
However, England winger Yarde touched down following a swift burst on 37 minutes, with Tim Swiel adding the extras to put the visitors in front for the first time.
Second-half tries from Kyle Sinckler and Tom Williams put the Dragons, who could only find a Robinson penalty in response, out of sight as Quins even prevented Welsh from picking up a bonus point.
The Exiles remain bottom of the table with one point from 12 matches, while Quins climb to eighth.
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