Disclosed herein are methods for determining the amount or activity of one or more luciferases and methods for measuring the luminescent signal generated by one or more luciferases in a sample, the methods comprising incubating the sample with a reactive substrate(s) of the luciferase(s) to be analysed and a reducing agent to inactivate a first luciferase, wherein the first luciferase, in its native form, is a secreted luciferase.
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