The present invention relates to an in vivo method of monitoring an analyte in a subject. This method involves providing an expression vector that encodes a biosensor molecule, the biosensor molecule comprising an analyte binding domain and a signal domain. The biosensor molecule produces a signal from the signal domain upon binding of the analyte by the analyte binding domain. The signal is detectable by a non-invasive means. The expression vector is introduced locally into in vivo cells of a subject under conditions effective to express the biosensor molecule in the cells. The signal from the expressed biosensor molecule is detected by a non-invasive means, thereby monitoring the analyte in the subject in vivo.