PRF1 and infection: We observed no significant difference between perforin-null, perforin-heterozygous, and perforin-WT mice pertaining to numbers or frequency of CD4 T cells (Fig. 1b, c), CD8 T cells (Fig. 1d, e), or NK cells (Fig. 1h, i) in the brains of mice with a 7-day TMEV infection.