X (エックス Ekkusu?), also known as X/1999, is a Japanese shōjo manga series created by CLAMP, a creative team made up by Satsuki Igarashi, Nanase Ohkawa, Tsubaki Nekoi, and Mokona. It premiered in Monthly Asuka's May 1992 issue and ran there until the magazine's editors showed concern with the increasingly violent stories and the series went on long-term hiatus in March 2003. The story takes place at the end of days, in the year 1999. The series follows Kamui Shirō, a young esper who returns home to Tokyo after a six-year absence to face his destiny as the one who will determine humanity's fate. The series is a foray into apocalyptic fiction, it combines elements from various end of the world scenarios, both secular and religious, with its own mythos.#1
Kadokawa Shoten collected and published the individual chapters in 18 tankōbon volumes, with five chapters published in the book "All about Clamp". Probably all but one or two final chapters have yet to be published.#2 It has been adapted into a series of audio dramas, a 1996 animated feature film and a television series in 2001 with the two last ones produced by Madhouse.