Estimation of S-phase fraction in tumor tissue sections by immunohistochemical staining of PCNA

We describe a method for measuring the size of the S-phase fraction in human tumor tissue sections using an antibody to PCNA (PC10). Although treatment with Triton X-100 before fixation extracted a large amount of PCNA from the cells even in frozen tissue sections, PCNA remained exclusively in S-phase cells. Immunohistochemical staining of PCNA after the detergent treatment allowed estimation of the S-phase fraction in solid tumors. The validity of the method was directly proven by double staining of bromodeoxyuridine (BrdU) and PCNA in HeLa cells. The PCNA-positive cells were identical with BrdUrd-positive cells after the detergent treatment. In contrast, almost all HeLa cells in the exponentially growing phase were positive for PC10 without treatment with Triton X-100.