Ahr is a river in Germany, a left tributary of the Rhine. Its source is at an elevation of approximately above sea level in Blankenheim in the Eifel, in the cellar of a timber-frame house near the castle of Blankenheim. After it crosses from North Rhine-Westphalia into Rhineland-Palatinate.

The aryl hydrocarbon receptor (AhR or AHR or ahr or ahR) is a protein that in humans is encoded by the AHR gene. The aryl hydrocarbon receptor is a ligand-activated transcription factor involved in the regulation of biological responses to planar aromatic (aryl) hydrocarbons. This receptor has been shown to regulate xenobiotic-metabolizing enzymes such as cytochrome P450.