CD5 and B-cell chronic lymphocytic leukemia: To investigate the in vivo therapeutic mechanism of PI3Kδi, we utilized the Eμ-Tcl1 Tg model, a gold standard for aggressive CLL.33, 34 Consistent with previous reports35, 36 our cohort presented with a CD5+ B-cell leukemia with phenotypic hallmarks of CLL, extensive splenomegaly and a median survival of 294 days (Supplementary Figures 1–2).