To search for novel regulatory regions, we examined the features of chromatin structure in the rat B29/Ig-b gene and its flanking regions by determining DNase I hypersensitive sites (DHS) in plasmacytoma-derived Y3 cells. Six Y3 cellspecific DHS were detected at ), promoter, +, +, +, and + kb. The DHS at +, +, and + kb were present in the intergenic region between B29/Ig-b and growth hormone (GH) genes and were mapped inside conserved sequences in rat and humans.