p53 status is the primary determinant of MDM2 cell kill in CLL. CLL samples (n=106) were enriched for CD19⁺ cells through negative selection and incubated for 36 hours with various concentrations of either MI-63 or Nutlin3. Samples were prepared for Annexin-V and PI staining and analyzed by flow cytometry, and the residual live and nonapoptotic cell fraction was calculated for each concentration by comparison with the untreated control incubation. (A) MI-63 assay results. Red indicates p53 sequence mutants; green, CLL cases with absent p53 expression; black, wild-type p53 status. (B) Nutlin3 assay results. Red indicates p53 sequence mutants; green, CLL cases with absent p53 expression; black, wild-type p53 status.