*2.4. Cell Proliferation Assay*

Bromodeoxyuridine (5-bromo-2-deoxyuridine, BrdU) is a synthetic nucleoside that is an analogue of thymidine. BrdU can be incorporated into the newly synthesized DNA of replicating cells (during the S phase of the cell cycle), substituting for thymidine during DNA replication. For the labeling of cells in the S phase, pregnan<sup>t</sup> mice (E13.5, E16.5, E17.5) obtained from timed mating were injected intraperitoneally with 10 mg BrdU (Invitrogen #000103) per 100 g of body weight. The brains dissected from E13.5, E16.5, and neonatal o ffspring (P0.5) were recovered in ice-cold PBS at pH 7.4 and fixed in 4% paraformaldehyde. Incorporation of modified nucleotide was detected by staining with anti-BrdU antibody (BU-20) as described previously [24].
