MUC21 and lung adenocarcinoma: Among the top highly expressed genes for each subtype (Figure 2b,c), claudin 2 (CLDN2), secretoglobin 3A2 (SCGB3A2), and mucin 21 (MUC21) have been well reported in the literature as potential diagnostic biomarkers in lung adenocarcinoma [24,25,26]; meanwhile, keratin family members (KRT6A, KRT6B, KRT6C, KRT14, KRT16) have also been reported to be upregulated only in lung squamous cell carcinoma [27,28,29,30].