TNF and viral infectious disease: In contrast, the expression of Cre in WT MEF cells did not affect the expression level of TAK1 or RIPK1 S321 phosphorylation in response to TNFα (Fig. 2b), suggesting the blocked RIPK1 S321 phosphorylation in TAK1F/F Cre MEFs was not a result of virus infection or Cre expression per se.