AHR and cancer: Thus, the accumulation of AHR in the nuclei of malignant tumors can be attributed to the following abnormalities: the aberrant activation of transcriptional regulators [such as protein kinase CK2, signal transducer, and activator of transcription 6 (STAT6) and NFκB] that enhance AHR expression (Belguise et al. 2007); obstructions in the negative feedback loop; and most importantly, constitutive AHR activation (even in the absence of ligands).