In the present study we examined gene expression and glucose-6-phosphate dehydrogenase (G6PD) activity in leukemic cells isolated from G6PD normal and deficient subjects. The results have shown that G6PD activity strongly increases in G6PD normal leukemic cells as well as in G6PD deficient leukemic cells when compared to peripheral blood mononuclear cells (PBMC). Higher levels of G6PD gene expression were observed in leukemic cells from G6PD deficient patients compared to G6PD normal. A similar pattern of gene expression was also observed for 3-hydroxy-3-methylglutaryl coenzyme A (HMGCoA) reductase. These results support the hypothesis that G6PD deficient cell, in order to sustain their growth, must respond to the low activity of their mutant enzyme with an increase in quantity through an induction of gene expression.