Apoptosis functions as a tumor suppressor through the elimination of cells with harmful mutations. BRUCE as one of the inhibitors of apoptosis proteins (IAP) suppresses the caspases 3, 6, 7, and 9 [
51], and facilitates degradation of apoptotic proteins such as caspase-9 and SMAC/DIABLO [
52,
53]. Nuclear and cytoplasmic expression of BRUCE has been observed in ESCC tissues. Advanced stage tumors had more levels of nuclear BRUCE protein expression in comparison with the normal squamous cells of the esophagus [
14]. P53, p16INK4a, and MDM2 are essential G1 cell cycle regulators which are associated with tumor progression [
54]. The p16 is an inhibitor of the cyclin D–depended protein kinase which is inhibitor of G1/S checkpoint [
55]. Methylation of p16INK4a promoter sequence is a critical mechanism for its inactivation and promotes the ESCC progression [
56]. P16 promoter hypermethylation has been observed among the ESCC cases in Northeastern Iran [
15]. Methylation status of the p16 promoter was evaluated using a MSP assay in 50 ESCC patients. There was a significant correlation between p16 protein expression and methylation. Moreover, significant differences were observed in methylation and loss of p16 protein expression between tumor and normal tissues highlighting the critical role of epigenetic among Iranian ESCC cases [
16]. P16 hypermethylation was observed in 73% of Iranian cases. Moreover, p16 hypermethylated ESCC patients had 59 and 36% of p16 hypermethylation in their blood and serum samples, respectively [
17]. It has been shown that there was an association between p16 promoter methylation and poor prognosis in Japanese ESCC patients [
57]. A mutation prevalence of 50% was observed among Iranian ESCC tumors which had significantly higher ratio of G: C to A: T transitions at CpG dinucleotides of P53 [
58]. Golestan province has the highest rate of somatic TP53 mutations with at least one mutation in 89.9% of the cases. There was also a correlation between mutation patterns and tea drinking habits in which wild-type TP53 and G: C to A: T transitions were observed among people who drink tea urgently. Transversion mutations were also observed among patients who prefer to drink tea with a delay. Majority of these missense mutations results in accumulation of mutant p53 protein [
18]. CpG transitions by deamination of 5-methylcytosine to thymine can be associated with chronic inflammation [
59,
60]. Chronic inflammation also promotes tumorigenesis [
61]. Generally, rates of CpG dinucleotides mutations shows that there is a probable correlation between inflammatory process and ESCC progression in Iran. The p53 is employed to protect cells against DNA-damaging agents such as cigarette smoking [
62]. The p21 protein as a DNA synthesis inhibitor is regulated by wild type p53 [
63]. To evaluate the probable correlation between P53/P21 and cigarette smoking, levels of p53 and p21 protein expressions were assessed among 80 Iranian ESCC patients. They observed p53 and p21 as important players in ESCC progression in Northeast of Iran in which accumulation of abnormal p53 in normal margins can be a predictor of tumor recurrence. Moreover, they have shown a converse correlation between survival and p21 expression among the ESCC cases. Therefore, p21/p53 protein expression can be introduced as useful prognostic markers among Iranian patients with poor clinical outcome [
64]. The p53 overexpression was correlated with tumor differentiation, tumor size, and overall survival in a sub population of Chinese ESCC patients [
65]. In contrast, there were not any correlation between p53 expression and clinicopathological features and prognosis among a group of Japanese ESCC cases [
66]. In the case of Iranian familial cases, previous reports have shown that the first-degree relatives had higher risks of ESCC in comparison with the relatives of unrelated controls (34% vs. 14%) [
33,
67]. Whole-Exome Sequencing (WES) was employed to assess the germ line mutations in nine Iranian ESCC families. Three out of four detected variants included amino acid changes within KCNJ12/KCNJ18 gene. KCNJ12 encodes a potassium channel protein controls the tumor growth by cell cycle arrest. Moreover, KCNJ12 is associated with NF-kB signaling pathway [
19]. We have also assessed the role of DIDO1 as an activator of Caspase 9 and 3 during the apoptosis process in a sample of Iranian ESCC patients. There was a significant correlation between DIDO1 expression and tumor depth of invasion. It was shown that the DIDO1 has an important role in primary stages of tumor progression [
20]. The CDKN2 is also a regulator of cell cycle. It has been shown that there was a correlation between rs10811661 and rs1333049 in CDKN2A/B loci and poor prognosis in Iranian ESCC patients in which the CC genotype carriers had a lower survival rates [
21].