TOKYO — Until recently, cell phone novels — composed on phone keypads by young women wielding dexterous thumbs and read by fans on their tiny screens — had been dismissed in Japan as a subgenre unworthy of the country that gave the world its first novel, "The Tale of Genji," a millennium ago.

Then last month, the year-end best-seller tally showed that cell phone novels, republished in book form, have not only infiltrated the mainstream but have come to dominate it.

Of last year's 10 best-selling novels, five were originally cell phone novels, mostly love stories written in the short sentences characteristic of text messaging but containing little of the plotting or character development found in traditional novels. What is more, the top three spots were occupied by first-time cell phone novelists, touching off debates in the news media and blogosphere.

The cell phone novel was born in 2000 after a home-page-making Web site, Maho no i-rando, realized that many users were writing novels on their blogs; it tinkered with its software to allow users to upload works in progress and readers to comment, creating the serialized cell phone novel. But the number of users uploading novels began booming only two to three years ago, and the number of novels listed on the site reached 1 million last month, according to Maho no i-rando.

The boom appeared to have been fueled by mobile-phone companies' decision to offer unlimited transmission of packet data, like text-messaging, as part of flat monthly rates. The largest provider, Docomo, began offering this service in mid-2004.

"Their cell phone bills were easily reaching $1,000, so many people experienced what they called 'packet death,' and you wouldn't hear from them for a while," said Shigeru Matsushima, an editor who oversees the book uploading site at Starts Publishing, a leader in republishing cell phone novels.

The affordability of cell phones coincided with the coming of age of a generation of Japanese for whom cell phones, more than personal computers, had been an integral part of their lives since junior high school. So they read the novels on their cell phones, even though the same Web sites were also accessible by computer. They punched out text messages with their thumbs with blinding speed, and used expressions and emoticons, like smiles and musical notes, whose nuances were lost on anyone over the age of 25.