G6PD promoter methylation in control and triploid cells. A. Map of the 457 bp G6PD promoter region containing the CpG sites we analyzed for methylation (numbered 1–52). B. Methylation patterns from an early passage culture of GM04939 XXX triploid fibroblasts. CpG sites are represented as either methylated (■) or unmethylated (□); a question mark indicates that methylation status could not be determined. Each clone represents the methylation pattern of a single chromosome, and the number of methylated CpG's in each clone is given in the right column; clones are ordered from lowest to highest methylation. C. Methylation patterns from a late-passage culture of GM04939 XXX triploid fibroblasts. The proportion of hypermethylated clones (Xi alleles) is decreased in later passage cells, in agreement with the decrease in the number of Xi's seen cytologically. D. Methylation patterns from a culture of GM04376 XXX triploid fibroblasts that has a preponderance of cells with one Xi; the methylation patterns we observed are consistent with this deficit in Xi's relative to Xa's.