PRF1 and neoplasm: While tumor‐infiltrating NK cells expressed higher levels of perforin in Tipe2ΔNK/ΔNK mice than in wild‐type mice, the elevated perforin expression by Tipe2 deletion was compromised upon further deletion of Tbx21 or Eomes, as in Tipe2ΔNK/ΔNK; Tbx21ΔNK/ΔNK mice and in Tipe2ΔNK/ΔNK; EomesΔNK/ΔNK mice (Figure 4D), confirming that both transcription factors are required for the improved NK cell antitumor effector functions in Tipe2ΔNK/ΔNK mice.