The present invention provides a method of treatment for cancer in a subject comprising the administration of a prodrug, wherein the subject is colonised with an obligate anaerobic microorganism capable of expressing a polypeptide having nitroreductase activity, wherein the polypeptide exhibits preferential reduction of CB1954 to a 4-hydroxylamine (4HX) derivative instead of a 2-hydroxylamine derivative; and is capable of reducing CB1954 to the 4HX derivative with a Km of less than 30 micromolar. The present invention further provides an isolated polypeptide, nucleic acids encoding said polypeptide, vectors and host cells comprising said nucleic acids and vectors and uses of the above in treating solid tumours.