Glucose-6-phosphate dehydrogenase (G6PD) deficiency is an X-linked incompletely 
dominant enzyme deficiency that results from G6PD gene mutations. Women 
heterozygous for G6PD mutations exhibit variation in the loss of enzyme activity 
but the cause of this phenotypic variation is unclear. We determined DNA 
methylation and X-inactivation patterns in 71 G6PD-deficient female 
heterozygotes and 68 G6PD non-deficient controls with the same missense 
mutations (G6PD Canton c.1376G>T or Kaiping c.1388G>A) to correlate determinants 
with variable phenotypes. Specific CpG methylations within the G6PD promoter 
were significantly higher in G6PD-deficient heterozygotes than in controls. 
Preferential X-inactivation of the G6PD wild-type allele was determined in 
heterozygotes. The incidence of preferential X-inactivation was 86.2% in the 
deficient heterozygote group and 31.7% in the non-deficient heterozygote group. 
A significant negative correlation was observed between X-inactivation ratios of 
the wild-type allele and G6PD/6-phosphogluconate dehydrogenase (6PGD) ratios in 
heterozygous G6PD Canton (r=-0.657, p<0.001) or Kaiping (r=-0.668, p<0.001). 
Multivariate logistic regression indicated that heterozygotes with 
hypermethylation of specific CpG sites in the G6PD promoter and preferential 
X-inactivation of the wild-type allele were at risk of enzyme deficiency.