hi colleagues, i am using three liver cancer cell lines( chang liver, hepG2 and SK-HEP-1) and i want to know which house keeping gene can be used to compare them. That is which internal standard will have the same expresion level in the three cell lines. I used GAPDH and B-actin but their expressions are different among the cell lines. Must i fine an internal stand with the same expression in the three different liver cell lines before my result can be valid? would it be right to normalize the expression of my target gene to a house keeping gene to compare its expression in different cell lines, from different origins but from the same tissue?