Evidence of chimerism threshold in allogeneic chimeras necessary for long-term engraftment. (A,B) The drop in the engraftment prevalence and chimerism level between 3 and 24 weeks of age for each group of prenatal chimeras is depicted (*P < .05, **P < .001). Data points for the chimerism level represent the mean value plus or minus SEM. (C) Sample flow cytometry dot-plots for donor chimerism assessment of 2 chimeric littermates (no. 8442 and no. 8424) with initial chimerism levels either above or below 1.8% at 3 and 6 weeks of age. (D) Plot of serial chimerism measurements in low-level chimeras (PB chimerism < 3.5%) illustrating significance of initial chimerism threshold in predicting the durability of long-term engraftment. Data for high-level chimeras (PB chimerism > 3.5%) are not plotted; however, all remained stable chimeras throughout the study period of 1 year. (E) Chimerism summary for congenic and allogeneic strain combinations demonstrating that the significance of the chimerism threshold is limited to the setting of immunologic disparity. Both allogeneic and congenic data are pooled from similar transplant dose cohorts. Prevalence of detectable chimerism throughout the study period is listed for animals either above or below 1.8% (*P < .01).