Activation of the Arf promoter during T-ALL development. Healthy syngeneic C57BL/6 mice received donor T cells derived from either control Arf^−/− or “knock-in” Arf ^Gfp/Gfp (also functionally null) thymocytes infected with an ICN1-CFP vector. (A) Cells from day-12 cultured thymocytes (left panels) or from spleens of moribund mice (right panels) were studied by FACS for coexpression of vector-coded CFP (ordinate) and cellular Arf-encoded GFP (abscissa). (B) Hematopoietic tissues from 17 such mice were similarly analyzed and contained a significant proportion of CFP-marked cells that coexpressed GFP (ordinate). Error bars indicate the SD from the mean.