Unlike CA199 and CEA, the positive rates of miR-21 in serum and PBMCs were considerably higher (stages I to IV and overall GC positive rates of serum miR-21 were 88.88%, 90.90%, 90.00%, 94.44%, and 88.00% and of PBMCs miR-21 were 88.88%, 81.81%, 90.00%, 94.44%, and 84.00%). Here, CEACAM5 is linked to gastric cancer.