HAAO and esophageal squamous cell carcinoma: We next explored the expression of 28 DE-TrpGs, from which bulk RNA-seq data analysis in different cell types showed that 18 DE-TrpGs (ALDH9A1, ALDH3A2, ADH1B, AOX1, MAOA, ECHS1, ALDH2, ACAT1, HAAO, ALDH7A1, HADHA, AKR1A1, CYP4X1, INMT, CYP2U1, CAT, HADH, and NMNAT1) were significantly expressed at the single-cell level, especially the downregulated expression of fibroblasts in ESCC tumor samples compared with normal samples (Figure 3H).