PIWIL1 and esophageal squamous cell carcinoma: Interestingly, squamous cell markers such as SALL4, NANOG, SOX2, BMI1, OCT3/4, HIWI, ABCG2, CD133, podoplanin, and ALDH1 also correlate with ESCC tumor stages, therapeutic resistance, relapse, and patient prognosis [34, 39].