First, flow cytometry studies confirmed that Gr-1high cells from the lungs of HKx31-infected mice at day 3 post-infection did not express markers characteristic of B cells (B220+), T cells (CD3ε+) or NK cells (NK1.1+) (data not shown), although intermediate levels of Gr-1 expression were observed on CD8+ T cells and, to a lesser extent, on NK cells and B cells recovered from the lungs of virus-infected mice (Fig. 4A). This evidence concerns the gene PTPRC and infection.