Mash-ups, combinations of two or more songs blended together to make completely different tunes, have existed in one form or another since almost the dawn of recorded music. But the modern era of mash-ups took hold in 2001, with one of the first such fusions being DJ Freelance Hellraiser's remix of Christina Aguilera's 'Genie in a Bottle' and the Strokes' 'Hard To Explain.' Hellraiser combined the gritty downtown New York guitars and instrumentation with the pop-diva vocals to create 'A Stroke of Genie-us,' and with that, the so-called mash-up was born. Since then, bedroom DJs and superstar producers alike have taken up the art form, with perhaps every conceivable genre of music -- and some inconceivable, no doubt -- mashed-up along the way. With that in mind, we've teamed up with our friends at Loudwire and Ultimate Classic Rock to come up with the following list of 15 Mash-Ups You Need to Hear. Check 'em out!

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