As well as maturation in cold storage, most lagers are also distinguished by the use of Saccharomyces pastorianus yeast, a "bottom-fermenting" yeast that also ferments at relatively cold temperatures. It is possible to use lager yeast in a warm fermentation process, such as with American steam beer; while German Altbier and Kölsch are brewed with Saccharomyces cerevisiae top-fermenting yeast at a warm temperature, but with a cold-storage finishing stage, and classified as obergäriges lagerbier (top-fermented lager beer).[2][3]