PNP and infection: Disruption of the purine nucleoside phoshphorylase gene (pnp) [38] or nucleoside transporter 1 (nt1) gene [39] in P. yoelii gives rise to severely attenuated blood-stage infections, and mice that had undergone an infection with these knockout parasites did not develop detectable patent blood-stage parasitemia after infectious mosquito bite or sporozoite challenge [38], [39].