Prei4, N-terminal CBM20 (carbohydrate-binding module, family 20) domain. Preimplantation protein 4 (Prei4) is a protein of unknown function that is expressed during mouse preimplantation embryogenesis. In addition to the N-terminal CBM20 domain, Prei4 contains a C-terminal glycerophosphoryl diester phosphodiesterase (GDPD) domain. The CBM20 domain is found in a large number of starch degrading enzymes including alpha-amylase, beta-amylase, glucoamylase, and CGTase (cyclodextrin glucanotransferase). CBM20 is also present in proteins that have a regulatory role in starch metabolism in plants (e.g. alpha-amylase) or glycogen metabolism in mammals (e.g. laforin). CBM20 folds as an antiparallel beta-barrel structure with two starch binding sites. These two sites are thought to differ functionally with site 1 acting as the initial starch recognition site and site 2 involved in the specific recognition of appropriate regions of starch.