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Clonal origin of mouse liver cell tumors.

The clonal origin of tumors was studied in a large series of liver cell tumors in mice. Tumors were induced with phenobarbitone alone or following N-nitroso-diethylamine administration in female mice heterozygous for the sparse-fur strain. In this strain, a histochemical technique can be used in heterozygotes to differentiate clearly between liver cells expressing the histochemically positive normal or the histochemically negative abnormal form of the X-linked enzyme ornithine carbamoyl transferase. Three hundred twenty-seven liver tumors in heterozygous female Spf mice were studied: 157 (48%) were uniformly negative, and 160 (49%) were positive (some with partial enzyme loss). One hundred fifty-four liver tumors in normal mice were studied; all were positive, with a frequency of partial enzyme loss similar to that seen in the heterozygotes. Ten (3%) of the tumors in the heterozygotes contained some separate groups of positive and negative cells, but no tumor was made up exclusively of such groups. Even the smallest recognizable tumors were made up of single-phenotype cells, which suggested that a polyclonal origin followed at a later stage by clonal selection was unlikely. It is concluded that at least 97% of the tumors were of single-cell origin, and that convincing evidence of a polyclonal origin was completely lacking. It is also concluded that the histochemical demonstration of an X-linked enzyme in tumors induced in female animals heterozygous for an abnormal form of that enzyme provides an extremely useful technique for the study of the origins of neoplasia.