It has been observed in the studies cited above that heterokaryons with the same number of nuclei coming from each donor can have different differentiation responses. Blau and Blakely (1999) suggested that the differentiation outcomes are regulated by a network of opposing factors. Within this framework, stochastic responses to cell fusion can readily be explained by slight differences in the quantities of differentiation factors contributed by each cell type, which tip the balance one way or the other. The network determining cell fate would be most sensitive to random noise when the factors are in roughly equal concentrations. The sensitivity to molecular noise of the networks proposed here would be interesting to study, as it has been proposed that some types of differentiation during development could have a stochastic aspect (for example in adipogenesis, Tchkonia et al., 2002, or hematopoiesis, Enver and Greaves, 1998).