Steady-state platelet survival data in mutant mice cast doubt on the multiple-hit model. Best fits (minimized sum of squared residuals) for populations and cohorts of platelets in (A) Bcl-x^+/Plt20, (B) wild-type, and (C) Bak^−/− mice. Data points are the mean of replicates (4 for Bcl-x^+/Plt20, 6 for wild-type and Bak^−/−), and error bars represent one SD. Arrows indicate the time of biotin injection to establish the cohort label (24 hours). The efficiency of labeling (∼ 90% for X488 and ∼ 60% for biotin) and the half-life of biotin were accounted for in the fitting procedure (“Methods” and supplemental data). (D) Monte-Carlo bootstrap resampling was performed for each genotype to generate estimates of the best-fit model parameters: hit rate, τ⁻¹, and number of hits, k (20 samples; horizontal line and error bars represent median and interquartile range). The differences between any 2 genotypes are statistically significant (P < .001, 2-tailed unpaired t test) for either parameter; however, the magnitudes of the differences are clearly greater for the rate of hits. (E) Model fits to population survival data with label scaled by the initial percentage and time scaled by mean life span extracted from the best fits (curves overlay, same shape). (F) Expected (theoretical) curves if the rate of hits were the same in Bcl-x^+/Plt20 or Bak^−/− mice as in wild-type, and the decreased or increased mean life span were instead the result of differences in the number of hits that could be endured (note the shape change). Numerical values for the best-fit model parameters are reported in supplemental Table 1.