Interestingly, although all eligible dogs demonstrated evidence of constitutive canonical NF-κB activity in malignant lymph node biopsies, suggesting an ABC phenotype, none of the malignant canine lymph nodes stained positive for MUM1/IRF-4, an NF-κB target gene that is over-expressed in ABC-DLBCL and a hallmark of this subtype in humans (data not shown). This evidence concerns the gene IRF4 and diffuse large B-cell lymphoma.