(CNN) -- Does "Fifty Shades of Grey's" love affair between a dominant man and vulnerable young woman feel a tad bit familiar?
It might if you're a tried and true fan of the "Twilight" series.
Welcome to the world of fan fiction, where fans tweak or add to existing series, novels and characters -- oftentimes with a steamy twist.
By now you've probably heard about the best-selling e-book "Fifty Shades of Grey," which was released in paperback Tuesday and will soon come to life on the big screen. The novel has readers, primarily grown women, clutching their e-readers under the covers, among other places.
"Fifty Shades" has been such a hit that Universal Pictures and Focus Features reportedly shelled out $5 million for the rights to E L James' titillating trilogy, which many sites are referring to as "mommy porn."
And while the early "Twilight" novels were decidedly chaste (at first, anyway), writers of fan fiction will tell you there's a huge desire for erotic novels.
Fan fiction grew popular in the 1960s thanks to "Star Trek" as fans developed their own story lines to swap with other devotees. And the genre has flourished, allowing fans to adapt beloved stories and tailor existing literature to fit their own preferences, said Francesca Coppa, an English professor at Muhlenberg College and founding board member of the Organization for Transformative Works.
That's exactly what James did with "Fifty Shades," which drew inspiration from the insanely popular series by Stephenie Meyer. Christian Grey and Anastasia Steele are loosely based on "Twilight's" Edward and Bella.
James, a TV executive, wife and mother based in London, took a young adult novel and spiced it up, said Sarah Weir, a contributing editor at Yahoo! Shine. Weir said that since a lot of young adult novels have such broad fan bases now, the success of "Fifty Shades" will probably be replicated more and more.
As both a writer and a reader, Coppa said she appreciates the freedom fan fiction offers.
"Typically I'm happier to write stories that have sex in them than stories that have violence in them," she said. "I like the custom-madeness of it. It's not just the same story with sex in it. But, often, the characters have changed in many ways."
"Star Trek" fans familiar with "Kirk/Spock" fan fiction, in which the two male characters engage in a homoerotic relationship, can attest.
"A lot of erotic (fan fiction) is actually queer," Coppa said. "A lot of people were writing gay and lesbian scenes at a time when there wasn't much out there."
It made sense to write these scenes around "Star Trek," she said, which featured a "very consciously diverse group of people. ... The one conspicuous absence was gays and lesbians. Fans rewrote those stories to put queer people at the table."
Coppa added, "The wonderful thing about fandom is that you realize you're not alone."
So many mainstream stories now are about women who have children and balance exciting careers. But if the buzz surrounding "Fifty Shades" is any indication, there are women out there who just want to feel like desired, sexual subjects, Coppa said.
James' novel puts them there, providing an outlet for a sort of creativity that hasn't been considered mainstream.
Sexual content is often shunned by the publishing world, said Los Angeles Times staff writer Carolyn Kellogg. "People have been really turned on by this book. It's so interesting that something considered dirty and second-rate is being embraced by the industry."
However, not all fan fiction is erotic.
"Sometimes fan fiction is about seeing more of a character," Coppa said. "Saying, I want to see her in different environments, or see her in a different time. ... That has a happened a lot with Sherlock Holmes, whether he's in the 1890s or ... contemporary London."
But not all authors are on board with fan fiction.
"The Wolf Gift" author Anne Rice wrote about the genre on her website: "I do not allow fan fiction. The characters are copyrighted. It upsets me terribly to even think about fan fiction with my characters. I advise my readers to write your own original stories with your own characters. It is absolutely essential that you respect my wishes."
However, other writers have embraced it, allowing such stories to expand the demographic of their original work.
Take Pottermore for example.
J.K. Rowling's website is a space for Harry Potter fans who don't want those adventures to end.
"With Pottermore, (Rowling) has said, 'Yes, you can come in and tell those stories the way you want to," Kellogg said. "I'm not sure there will be a lot of other 'Fifty Shades of Grey,' but the way people deal with fan fiction may be shifting in a real and significant way."