S1 No. 1 Style (エスワン ナンバーワンスタイル) or just S1 (エスワン) is a Japanese producer of adult videos (AV) which has its offices in Tokyo's Meguro Ward. The company is best known for retailing its adult videos under the Fanza brand, an affiliate of DMM. In the past 15 years since its inception, S1 has risen to become one of the most prominent movie studios in the Japanese adult entertainment scene, producing over 5,100 movies to date.