Mollicutes is a class of bacteria distinguished by the absence of a cell wall. The word "Mollicutes" is derived from the Latin "mollis" (meaning "soft" or "pliable"), and "cutis" (meaning "skin"). Individuals are very small, typically only 0.2–0.3 μm (200-300 nm) in size and have a very small genome size. They vary in form, although most have sterols that make the cell membrane somewhat more rigid. Many are able to move about through gliding, but members of the genus "Spiroplasma" are helical and move by twisting. The best-known genus in the Mollicutes is "Mycoplasma".