An elegant experiment conducted by Fialkow et al. (1967, 1981) using patterns of inactivation in X-linked genes, showed that erythrocytes and myeloid cells in female CML patients with heterozygous X-linked glucose-6-phosphate dehydrogenase (G6PDH) had the same single isoenzyme type for G6PDH in contrast to normal cells, which were heterogeneous. The gene discussed is H6PD; the disease is chronic myelogenous leukemia, BCR-ABL1 positive.