Infrequent GSI-resistant primary human T-ALL samples include both PTEN-positive and -negative cases. In vitro analysis of 2 primary human T-ALL samples (D135 and M82) maintained briefly (2 days) on MS5-DL1 stromal feeders and then treated with GSI (1μM compound E) versus DMSO vehicle for 4 days. Cells were analyzed as in Figure 4 for proliferation (A), cell size (B), and viability by light scatter (C). PTEN protein expression was assessed by intracellular flow cytometry (D). FSC indicates forward light scatter; SSC, side scatter.