Lineage specificity of p53 accumulation, p21 levels, and cell cycle arrest. (A) Levels of p53 were determined by intracellular flow cytometry in cells expressing control (luciferase), RPS14, or RPS19 shRNAs. Lineage-specific activation of p53 protein was determined by staining for erythroid (CD71), megakaryocytic (CD41a), and myelomonocytic (CD11b) cell surface markers. (B) Levels of p21 were determined by intracellular flow cytometry in cells expressing control (luciferase), RPS14, or RPS19 shRNAs. Lineage-specific activation of p21 protein was determined by staining for erythroid (CD71) and myelomonocytic (CD11b) cell surface markers. (C) Primary human bone marrow cells were infected with control or ribosomal gene shRNAs and allowed to grow and differentiate over 5 days in the presence of cytokines supporting erythroid and myeloid differentiation. Cells were then labeled with BrdU, 7-AAD, and lineage markers (CD11b and CD71). Results in each bar graph are the composite data from 3 independent experiments performed in triplicate (mean ± SEM). *P < .05. **P < .01.