The serum concentration of Smad4 was negatively correlated with the degree of lymph node metastasis of NSCLC (N0–N1: 157.67 ± 26.65 ng/L; N2–N3: 88.24 ± 15.28 ng/L) (P < 0.001) (Table 3; Fig. 2e). Here, SMAD4 is linked to non-small cell lung carcinoma.