Figure 1: Phase 0 response to xenobiotics is activation of a transcriptional response by xenobiotic responsive transcription factors. Phase I-III detoxication is well documented and relatively well defined as oxidative metabolism, conjugation, and transport, respectively. Phase 0 xenobiotic response is defined as the transcriptional response of and initial acclimation of the cell to xenobiotics leading to increased phase I-III detoxication through gene regulation. R/TF = receptor/transcription factor.