As shown in Fig. 5A, the serum TK1 activity was significantly higher in the lymphoma (p <  0.0005), suspected lymphoma (p <  0.02) and tumour-free with concurrent diseases (p <  0.03) groups than in the controls without concurrent diseases, and there was a significant difference between the lymphoma group and the tumour-free group with concurrent diseases (p <  0.0006). Here, TK1 is linked to lymphoma.