The effect was ultimately thought to be due to the presence of a relatively small number of a particular kind of active cell, denoted as a

stem cell

. For the record, the

Academic Press Dictionary of Science and Technology

defines a stem cell as “a cell, capable of both indefinite proliferation and differentiation into specialized cells, that serves as a continuous source of new cells for such tissues as blood and testes.” Cancer biologist Robert Weinberg of the Whitehead Institute, Cambridge, Massachusetts, and discoverer of the first human oncogene, is quoted in the article as follows. “Within a tumor mass, there is only a small population of cells that can spawn more tumor; other cells in the tumor cannot.”