CpGs at the -3.9 kb enhancer are demethylated with different kinetics during in vitro differentiation. Genomic DNA prepared from ES cells, freshly prepared mouse embryonic fibroblasts (EFs), and purified B cells from transgenic mice as well as from cells undergoing differentiation, was subjected to quantitative bisulfite sequencing as outlined in “Materials and methods.” (A) Sequence of the -3.9 kb enhancer and position of CpGs. (B) CpG methylation level in the different cells as indicated.