Figure 1: Predicted composition of a cell culture after expansion of a single cell according to the mode of cell division. (A) By symmetrical division a non-GSC (green oval) will produce a culture composed of only non-GSCs. (B) A GSC (red oval) will also produce only GSCs and, non-GSCs may arise only by differentiation of GSCs (box) resembling asymmetrical division. (C) Asymmetrical division of a non-GSC does not occur by defi nition. (D) Clonal expansion of a single GSC will produce a cell culture containing increasing number of non-GSC and only one GSC unless the GSC at some point divides symmetrically (box).