You can learn everything you need to know about a person by finding out who their favorite Backstreet Boy is, and Meghan Trainor just shared hers with the world.

The Thank You singer posted a selfie with A.J. McLean on Instagram yesterday (May 17) after their performance together at ABC’s Greatest Hits concert. “Dream come true,” wrote Trainor, while McLean posted the same photo with the caption, "My new bestie. Loved performing with this amazing talent. Girl we gonna make history."

Trainor met all five of the Backstreet Boys during the Los Angeles event, but only one of them made it into her selfie. She’s an A.J. girl, which proves what we already know about her: She's got a bit of a rebellious streak and she isn’t afraid to show it.

Now that they’re best friends, will McLean and Trainor cook up something in the studio together? Could a BSB/M-Train collab be rolling down the tracks? We’ll see.