Cellulose 1,4-ò-cellobiosidase (reducing end) (, CelS, CelSS, endoglucanase SS, cellulase SS, cellobiohydrolase CelS, Cel48A) is an enzyme with systematic name 4-beta-D-glucan cellobiohydrolase (reducing end). This enzyme catalyses the following chemical reaction
The CelS enzyme from Clostridium thermocellum is the most abundant subunit of the cellulosome formed by the organism.