Not long ago, fan fiction was considered by the publishing world as little more than the literary equivalent of an annoying copycat little brother. But what was once viewed as either uncreative, a legal morass of copyright issues, or both, is now seen as a potential savior for a publishing industry still finding its moorings in the age of digital media.

This sea change can be attributed almost entirely to Fifty Shades of Grey, the E.L. James erotic romance that began life as Master of the Universe, a serialized riff on Edward and Bella from Twilight, first published on fanfiction.net. As it gained popularity, James took it off the board, changed the names of her characters, and sold it as an ebook from a small Australian publishing company. Eventually Vintage Books, a subdivision of Random House, picked it up and Fifty Shades of Grey would end up spending over 100 weeks on the New York Times bestseller list, with the trilogy selling north of 100 million copies, and a blockbuster film adaptation on the horizon.

Traditional publishing houses have been forced to catch up to a suddenly mainstream genre once considered purely niche. “Fan fiction has absolutely become part of the fiber of what we publish,” Gallery Books publisher Jennifer Bergstrom told The Washington Post in October. “This is changing at a time when traditional publishing needs it most.”

Twilight has proved in recent years to be one of the most fertile grounds for fan fiction, begetting not only Fifty Shades, but other trilogies such as The Beautiful Bastard and the Gabriel books, among others.

Other popular sub genres of fan fiction include “real person fiction,” which star thinly disguised actual people—the lads from One Direction are particularly popular here—as is “slashfic,” a genre that focuses on characters having homosexual relationships, and has its roots in fans reimagining the Kirk/Spock dynamic from Star Trek. Others imagine “alternate universes” or AUs, often where supernatural or magical characters are forced to live the mundane lives of everyday people. The gang from the Harry Potter appears regularly in all of the above.

It makes sense that J.K. Rowling and Twilight-creator Stephenie Meyer are among the most permissive authors when it comes to allowing other writers to borrow their characters. Others, like Anne Rice and George R. R. Martin, abhor the concept. As the A Song of Ice and Fire author put it, “It's a lazy way to go when you're just taking my characters.”

It can also be a confusing pond for publishers to wade into. One person’s Death Comes to Pemberley—a bestseller in which crime writer P.D. James imagines a mystery set in the world of Pride & Prejudice—is another person’s copyright infringement. “The line is not clear between inspiration and fan fiction,” said Ashleigh Gardner, content head of Wattpad, one of the more popular online forums for fan fiction along with fanfiction.net, and archiveofourown.org. “It’s very much about how the author self-identifies their work.”

The smoothest road for fan fiction creators is to use characters in the public domain, which is why you see so many inspired by the works of Jane Austen. (Our favorite? Fifty Shades of Mr. Darcy by William Codpiece Thawkery)

As for the modern stuff, publishers tend to let it live as long as there aren't major profits involved, choosing to look at fan fiction as a way of marketing their books and building a following. As Rebecca Tushnet, a copyright expert at Georgetown Law School put it to the Wall Street Journal, "It's clear that it's not good business to sue your customers.”