TK1 and lymphoma: Biomarker studies, including γ-H2AX cytometry analysis, serum thymidine kinase 1 activity and pretreatment D-dimer level, revealed that spontaneous occurring lymphomas in dogs share clinical and biological similarities with their human counterparts, and provide opportunities to optimize drug development by generating relevant biomarker data unavailable with human xenografts or genetically engineered mice models [4–6].