St Helens (18) 30
Tries: M Smith, Cunningham, Fozzard, Graham, Gardner
Goals: Lyon (5) Harlequins (8) 24
Tries: Heckenberg, T Smith, Bradley-Qalilawa, McLinden
Goals: Paul (4)
Brian McDermott's reign as Harlequins coach began with a narrow defeat at table-toppers St Helens.
Two goals and a Daniel Heckenberg try put Quins ahead before tries by Matty Smith, Keiron Cunningham and Nick Fozzard gave Saints the advantage.
Tyrone Smith cut the lead before James Graham restored Saints' points cushion.
Nick Bradley-Qalilawa pulled one back before Adie Gardner pushed Saints out of reach, but Mark McLinden's try ensured Quins scored the final points.
St Helens coach Daniel Anderson on Matty Smith: "He got a rollicking from me because he made half a dozen errors in the game.
"You can do that in the academy level, but you get burned in the top grade and we got burned.
"It was not just his mistakes - it was the whole team - and we must have made 20 mistakes tonight."
Harlequins coach Brian McDermott: "I'm not concerned about being dragged into the relegation dogfight. I'm looking towards the top six.
"As a coach there are areas of the team I want to improve and get them to play my way.
"There are areas we're good at and can enhance. If we focus on that then other issues will take care of themselves."
St Helens: Wellens, Gardner, Lyon, Gilmour, V. Anderson, Pryce, Smith, Cayless, Cunningham, P. Anderson, Wilkin, Graham, Hooper. Replacements: Roby, Fozzard, Hardman, Fa'asavalu.
Harlequins: McLinden, Smith, Gafa, Lucky Luisi, Bradley-Qalilawa, Dorn, Leuluai, Mills, Randall, Heckenberg, Purdham, Hopkins, Paul. Replacements: Budworth, Weisner, Mbu, Tookey