The X stands for Xtreme in this modern reboot of PUMA's 1980s Running System sneaker that pushes the limits of proportion, texture, shape and even comfort. High-shine finishes and mixed components top the super-chunky sole for a look that's inspired by science fiction and grounded in tomorrow. Style Name: Puma Rs-X Sci-Fi Sneaker (Women). Style Number: 5883359. Available in stores.