Molecular biology's lessons about Toxoplasma development: Stage-specific homologs

Within intermediate hosts (such as humans), the protozoan parasite Toxoplas ma gondii has two life cycle stages: a rapidly replicating form called a ta chyzoite and a slowly grouting, quiescent form called a bradyzoite. Recentl y, molecular biology studies have shown that tachyzoites and bradyzoites ex press a number of homologs (ie. evolutionary related genes) expressed exclu sively in one ou the other stage. Here, Laura Knoll and John Boothroyd desc ribe examples of how these stage-specific homologs were discovered, and spe culate about their regulation and functional significance.