Our data are the first to document the expression of T lymphocyte-associated factor expression, including the co-inhibitory ligands (PD-L2, Gal9), transcription factors (GATA3, TBX21, and RORγt), chemokine receptors (CXCR3 and CCR2), and inflammatory cytokines (IL-4, IL-12p40, and IL-17) relevant to canine mammary tumors. The gene discussed is PDCD1LG2; the disease is breast cancer.