PLK1 and infection: Δgra9 vaccination effectively elicited immune responses which conferred absolute protection from subsequent lethal type I RH, type II PLK wild-type tachyzoites or type II PLK cysts challenge infections in mice, suggesting that the mutant Δgra9 of T. gondii type II PLK strain is a potential live-attenuated vaccine candidate against acute and chronic toxoplasmosis.