**2.5 Comparative genomic hybridization**

Genomic DNA from male spleens was prepared using a standard Proteinase K/ammonium acetate/isopropyl alcohol precipitation protocol as described (Bilger et al., 2004), except DNA was treated with RNase A, which was then precipitated with ammonium acetate before the final DNA precipitation. Genomic DNA of either line 1R5 or inbred C3H mice was then labeled and mixed with control genomic B6 DNA, for dual-color CGH, and hybridized to the MM 8\_WG\_CGH\_1of8 chip. This array has 50-75 nucleotide probes spaced at an average of 650 bp, covering all of Chr 1 and Chr 2 through position 123959568. Results were analyzed by Nimblegen CGH Services, using the CGH-segMNT algorithm (Roche Nimblegen, Madison, WI).
