A method is provided for measuring in vivo in a transgenic non-human multicellular organism the activity of a cellular enzyme, which organism is transgenic by virtue of comprising one or more nucleic acid constructs encoding a binding domain and a binding partner thereof, wherein: (i) the binding domain and/or binding partner comprise a site subject to post-translational modification by the cellular enzyme; (ii) modification of the site by the enzyme affects the interaction between the binding domain and the binding partner; and (iii) the binding domain and the binding partner each comprise a detectable label such that when the binding domain and binding partner interact, a detectable physical characteristic of one or both of the labels is altered, which method comprises measuring the interaction between the binding domain and the binding partner by measuring changes in said physical characteristic in one or more cells of the transgenic organism. A transgenic non-human multicellular organism is also provided.
