The Spearman correlation analysis (Fig. 4C-I) demonstrated that there was negative relationship between IL4I1 expression and MAOA (r = -0.51), MAOB (r = -0.15), NIT2 (r = -0.32), HPD (r = -0.33), MAT2A (r = -0.35), and there was positive relationship between IL4I1 expression and AOC1 (r = 0.28,), DDO (r = 0.17) in the co-expressed genes in thyroid cancer from TCGA dataset (all p values ≤ 0.001). This evidence concerns the gene HPD and thyroid gland carcinoma.