The USWNT captain helped Lyon preserve their perfect start to the Division 1 Femenine campaign.
Horan fired in her team's third goal of the game after just 23 minutes and then rounded off the scoring with two late in the second half.
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THE BIGGER PICTURE
Lyon have now won all four of their matches in the French top-flight and sit top of the table, level on points with Paris FC. Saint-Etienne, meanwhile, are bottom of the league with just one point from four games.
DID YOU KNOW?
Horan had not scored for Lyon this season before Saturday's match. The 29-year-old is already closing in on last season's tally, however, as she struck five times in 15 matches in the league.
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WHAT HAPPENS NEXT?
Horan and her Lyon co-stars' next match is on October 22 against Reims, whose game against Paris Saint-Germain was abandoned due to fireworks exploding above the pitch.