Protease-activated receptor (PAR) 1, PAR3, and PAR4 are considered "thrombin receptors" because thrombin specifically cleaves the extracellular N-termini of the receptor to unmask a new amino acid terminus, which in turn acts as a peptide ligand by binding intramolecularly to the body of the receptor. Among those 3 family members, PAR1 is the predominant thrombin receptor. Although the thrombin-mediated regulation of clot formation has been studied extensively over the past decades, the possible role of thrombin in tumor metastasis via PAR1 has only recently received attention and is briefly discussed herein.