DDAH1 and head and neck neoplasm: Tumor and normal tissue microarray data (NPC, GSE12452, GSE53819; head and neck tumor, the Cancer Genome Atlas) revealed that AK4 was the only gene that was upregulated in both NPC and head and neck tumor samples compared to normal samples among the gene signatures (AK4, CPAMD8, DDAH1, and CRTR1) (Fig. 1A, B, C; Supplementary Fig. 1A, B, C).