We're revealing the five most popular songs in the Billboard Hot 100 Pop Singles chart, for the week ending September 17, 2016. Way back in 1988, Rob Base and DJ EZ Rock had a hit single with “It Takes Two.” How right they were: duos are all the rage in the current Top Five. Number 5: Chainsmokers Featuring Daya " Don't Let Me Down " Last week, twenty one pilots made Hot 100 history; this week, The Chainsmokers double down as “Don’t Let Me Down” featuring Daya rebounds two slots to fifth place. Daya, 17, is about to drop her first full-length album. Titled Sit Still, Look Pretty – after the hit single of the same name – it arrives on October 7. Number 4: Sia Featuring Sean Paul "Cheap Thrills" Here’s another duo for you: Sia and Sean Paul retreat a slot to fourth place with their former champ “Cheap Thrills.” This is the most-heard song on U.S. radio. It tallies 148 million airplay audience impressions on the Billboard Radio Songs chart, leading that list for a fifth week. Number 3: Twenty One Pilots " Heathens " Taking over third place is twenty one pilots, reaching a new Hot 100 high with “Heathens.” The Ohio duo is touring the world in support of its Blurryface album, and starting on January 17, Jon Bellion will join the North American leg. Jon’s a rising singer-songwriter: he wrote the chorus for Eminem and Rihanna’s smash hit “The Monster,” and earlier this year, his debut album The Human Condition hit fifth place on the Billboard album chart. Number 2: Major Lazer Featuring Justin Bieber & MO "Cold Water" Major Lazer is in fine condition this week, as “Cold Water” holds in the runner-up position. The video is out now, but someone is missing… MO is seen onstage, but Justin is conspicuous in his absence. The Biebz is currently on the European leg of his own world trek. Their schedules just didn’t mesh. Number 1: The Chainsmokers Featuring Halsey "Closer" Everything’s meshing nicely for the Chainsmokers and Halsey, who retain the Hot 100 title for a third week with “Closer.” On Sept. 18, Halsey will appear with Pentatonix as the Formula 1 Grand Prix in Singapore, while Oct. 16 brings an appearance at the Forbes Under 30 Music Festival in Boston. She’ll co-headline with Jason Derulo. That’s it for now, but be sure and join us for a new countdown next week.