CST1 and cyst: When exogenous 150 μM biotin was supplemented to the bradyzoite-inducing medium, the BirA* tag fused to MAG1, MCP4, BPK1, CST1, and GRA6 displayed biotinylation activity as an increase in streptavidin signal that was observed at the cyst wall compared to that in the Pru strain under IFA (Fig. 1B).