In vitro growth of LKS cells. CD34^-LKS cells were isolated from the recipients of p21^-/- primary cBMT (n = 3) or quaternary p18^-/-p21^-/- cBMT (n = 4). CD34^-LKS cells (100) were sorted into 96-well plates and cultured in IMDM supplemented with 100 ng/mL SCF, 50 ng/mL Flt-3, and 25 ng/mL TPO. Cell numbers were counted at 3 time points during the culture. The growth curves were plotted based on the means ± SD of triplicate cultures. An aliquot of the differentiated cells were lysed for the semiquantitative PCR analysis as described in Figure 2. The inserts in the graph are the PCR results showing the altered genotypic representation (knockout vs wild-type). Based on the single-cell analysis (Table 1), the initial ratios of p21^-/- and p18^-/-p21^-/- in the starting CD34^-LKS cells were 63.3% and 78.9%, respectively. No differentiation block was observed in both groups based on the cell morphology under a microscope.