The monoclonal theory was first proposed based on the analysis of female cancer patients heterozygous for the X-linked glucose-6-phophate dehydrogenase (G-6-PD) locus: a given tumor mass was shown to express enzymes of either type A or B, whereas both A and B were found in normal tissues, suggesting that various types of tumors, including colon carcinomas, are monoclonal13. Here, G6PD is linked to cancer.