ALDH1A3 and nasopharyngeal carcinoma: In order to find out the possible mechanism of the HOX gene activation, we analyzed the expression of the potential activators of HOX genes and found the upregulation of RALDH2 and RALDH3 genes (Figure 6) in NPC and TDN from PD2 and PD3 patients that encoding the main enzymes in the synthesis of retinoic acid (RA), which is an activator of expression of predominantly the 3′ region of HOX cluster genes [34,35,36,37,38].