Two former employees of Toyō Itō, Kazuyo Sejima and Ryue Nishizawa formed a collaborative partnership in 1995 called SANAA. They are known for creating lightweight, transparent spaces that expose the fluidity and movement of their occupants. Their Dior store in Shibuya, Tōkyō, in 2001 was reminiscent of Itō's Mediatheque, with cool white acrylic sheets on the external facade that filter the light and partially reveal the store's contents. Their dynamic of fluidity is demonstrated by the Rolex Learning Centre at École Polytechnique Fédérale de Lausanne, completed in 2010. This building has an undulating floor plane set under a continuous concrete shell roof that was poured in one go over two days. The plan is like a biological cell punctuated with tables and courtyards alike. In 2009 they designed the Serpentine Gallery Pavilion in London that comprised a reflective, floating aluminium roof supported by slender columns.