The sense of bitter taste is mediated by a group of bitter taste receptor proteins that reside on the surface of taste cells within the taste buds of the tongue. These proteins are 7-transmembrane domain, G protein-coupled receptors that are encoded by the TAS2R gene family (see TAS2R10; 604791), which contains at least 25 functional genes (Kim et al., 2005).

Humans worldwide display a bimodality in sensitivity to the bitter taste of PTC, with approximately 75% of individuals perceiving it as intensely bitter, whereas the rest perceive it as tasteless. This difference has been the basis of study of taste perception in humans for over 70 years. Kim and Drayna (2004) provided an historical review of the subject.

Propylthiouracil (PROP) and PTC are members of a class of compounds known as thioureas. The compounds carry the chemical group N-C=S, which is responsible for their characteristic bitter taste (Bartoshuk et al., 1994; Drewnowski and Rock, 1995).