Choosing human HSC replication rates for simulations. For each simulated person, we selected the replication rates for HSCs expressing the maternal X-chromosome and for HSCs expressing the paternal X-chromosome at random from a distribution with common mean λ. The SD of this distribution was defined such that the ratio between values 2 SDs above and below this mean would be equal to the ratio between λ_d and λ_G in cats (the horizontal line). If the variability in humans is smaller than assumed here, there should be a narrower range of possible values for λ. We suspect that this more stringent limit (which would result in a narrower range of ∼ 1 per 40 weeks) is indeed probable.