Ordered Ig rearrangements during B cell differentia- tion. Both the unrearranged H and L chain loci ( Fig. 1 ) are silenced in non-B lymphoid cells, as well as in early B cell precursors. In these cell types, the IgH and IgL chain loci are maintained in an extended conformation at the nuclear periphery, which is regarded as a repressive environment (7) . All genes are packaged in a basic nucleosome-repeat structure, but alterations in histone-tail modifications can greatly influence both local and higher-order chromatin conformation