Expression (A) and enhanced kinase activity (B) of the p210^bcr/abl transgene product in the thymi of the two founder mice, 4-3 and 4-5 that developed ALLs. The expressed and phosphorylated p210^bcr/abl transgene products are indicated by arrows and the positions of protein markers are shown on the left. The thymi of nontransgenic mice were used as negative controls (C).