Fig. 1. Schematic maps indicating the organisation of the MHC, EDC and 6p24 genomic regions and the location of the genomic probes used for FISH analysis. (A) Ideograms show the positions of the EDC on chromosome 1 and the MHC and 6p24 regions on chromosome 6. The EDC contains 27 genes and spans a genomic region of 2.5 Mb, it is mainly composed of genes involved in keratinocyte differentiation and can be visualised using genomic probes to the small proline rich region (SPRR). The 6p24 region is relatively gene-poor, with the 500 kb subsection studied flanked by only two known genes. The MHC is divided into three classes and contains many genes involved in the immune response. It is one of the most gene-rich regions of the genome, with 224 gene loci identified over a genomic region of 3.9 Mb. A more detailed map of the MHC class II and adjacent centromeric region is shown in (B), with the relative positions of the LMP/TAP, DAXX, BAK and TCP11 regions indicated. A number of genes mapping to this region are shown, for a more comprehensive list refer to the Sanger Centre website (www.sanger.ac.uk). The region deleted in the 721.174 cell line is shown as a solid bar and covers a large proportion of the MHC class II region from the DRA gene up to the LMP/TAP region.