Endo-1,4-mannanase, a crucial enzyme for depolymerization of seed galactomannans and wood galactoglucomannans. Random hydrolysis of (1->4)-beta-D-mannosidic linkages in mannans, galactomannans and glucomannans. Belongs to the glycosyl hydrolase 5 (cellulase A) family. Extended N-terminus.