However, in stark contrast to WT mice, they developed sustained high parasitemias during the chronic phase of infection (e.g., parasitemias of 17% in Il21-/- and 23% in Il21r-/- mice compared with less than 0.04% in WT C57BL/6 at day 36 post-infection) (Fig 1 B). This evidence concerns the gene IL21 and parasitic infectious disease.