Mice lacking Sμ tandem repeats maintain RNA polymerase patterns but exhibit histone modification pattern shifts linked to class switch site locations

► Histone H3 acetylation and trimethylation mark CSR accessible regions in B cells. ► Sμ histone H3 marks depend on flanking but not underlying DNA sequences. ► Histone H3 dimethylation marks match with downstream boundaries of Sμ CSR. ► Increases in RNAPII loading in the Eμ/Iμ region are linked to initiation of CSR. ► Sequences just upstream of the Sμ tandem repeats control Sμ RNAPII loading.