Patent ID: 7635598

Claim:
A mixture comprising: (a) a probe comprising a fluorophore and a quencher joined by a linker, wherein said quencher is a substrate for an azoreductase, wherein said quencher and said linker have the structure: wherein R 4 and R 5 are members independently selected from substituted or unsubstituted alkyl and substituted or unsubstituted heteroalkyl; R 7 , R 8 and R 9 are members independently selected from H, —NR 10 R 11 , substituted or unsubstituted aryl, nitro, substituted or unsubstituted C 1 -C 6 alkyl, substituted or unsubstituted C 1 -C 6 heteroalkyl, and OH wherein R 10 and R 11 are independently selected from H, substituted or unsubstituted alkyl and substituted or unsubstituted heteroalkyl; a is an integer from 0to 4; b is an integer from 0 to 4; and c is an integer from 0 to 4; and X 1 and X 2 are members independently selected from —CH 3 , and a bond to said fluorophore, and at least one member selected from X 1 and X 2 is said bond to said fluorophore; and (b) a cell expressing said azoreductase, said cell comprising a vector comprising a nucleic acid coding said azoreductase operably linked to a promoter.