BrdU Incorporation Assay to Analyze the Entry into S Phase

5-Bromo-2'-deoxyuridine (bromodeoxyuridine (BrdU)), a modified nucleotide and analog of thymidine, is commonly used for detecting proliferating cells. For detection, an anti-BrdU antibody (probe) with a fluorescent dye is applied to bind the BrdU label after DNA denaturation. In this protocol, we provide the BrdU labeling method for both in vitro and in vivo studies, along with immunocytochemistry (ICC)/immunofluorescence (IF) and immunohistochemistry (IHC) staining procedures, respectively. Multicolor staining is also presented as an option to detect the co-distribution of two or multiple antigens in the same sample, making it possible to visualize the location of different molecules at the same time.