Carboxymethylenebutenolidase (CMBL), also known as 4-carboxymethylenebut-2-en-4-olide lactonohydrolase, maleylacetate enol- lactonase, dienelactone hydrolase, and carboxymethylene butenolide hydrolase, is a hydrolase specially belonging to the family of hydrolases. It maily acts on carboxylic ester bonds. CMBL is a human homolog of Pseudomonas dienelactone hydrolase involved in the bacterial halocatechol degradation pathway. The ubiquitous expression of human CMBL gene transcript in various tissues was observed. CMBL was demonstrated to be the primary olmesartan medoxomil (OM) bioactivating enzyme in the liver and intestine. The recombinant human CMBL expressed in mammalian cells was clearly shown to activate OM. The recombinant CMBL also converted other prodrugs having the same ester structure as OM, faropenem medoxomil and lenampicillin, to their active metabolites. CMBL exhibited a unique sensitivity to chemical inhibitors, thus, being distinguishable from other known esterases.