MMP1 and esophageal squamous cell carcinoma: Considering the effects of using different substances and MMP1 level together as predictors, we found heavy smokers (more than 20 pack-years) who had MMP1 levels >9.67 ng/mL to have 61.4 times the risk of ESCC (AOR = 61.4, 95% CI = 10.7–356.7) compared with non-smokers with lower MMP1 levels.