He also praised the group's artwork and storytelling style as having "struck a strong chord with male and female manga readers". The group was placed third after the winner for the Shogakukan Manga Award in the Children's category in 1999. Their work "Cardcaptor Sakura" won the Seiun Award for best manga in 2001. Almost 100 million Clamp "tankōbon" copies have been sold worldwide as of October 2007. Various of their selling series include "xxxHolic" and "Tsubasa: Reservoir Chronicle" that have sold over eleven million and twenty million volumes, respectively. When asked about the universal popularity of Clamp's works, John Oppliger of AnimeNation stated that although it is "not based on originality [or] their artistic skill", they possess a distinct style that "perfectly mesh[es] the conventional attributes of shōnen and shōjo manga". He also pointed out that Clamp often "recycles" characters from their own earlier works, which gave rise to "a loosely defined 'Clamp Universe' that gives much of their work a unifying tone", and creates "absorbing, complex narratives that appeal to both male and female readers". All these factors result in "a cult following devoted to anything and everything the group publishes". "" authors stated that "whatever Clamp are on, we'd like some". Helen McCarthy in "500 Essential Anime Movies" stated that Clamp's works "are among the most successful manga and anime with Western fans".