The cells in early mouse embryos are generally thought to have equivalent developmental properties until at least the eight-cell stage. On p. 479,Piotrowska-Nitsche and colleagues challenge this idea by showing for the first time that four-cell stage mouse blastomeres have different developmental properties that depend on their embryonic origin. In most two-cell mouse embryos, one cell divides meridionally and the other divides equatorially or obliquely with respect to the second polar body. The researchers report that chimaeras made entirely of specific equatorially or obliquely derived four-cell stage blastomeres show developmental abnormalities - those made from the most vegetal blastomeres are most severely affected. By contrast,chimaeras made from meridionally derived blastomeres develop normally. However, when individual blastomeres are surrounded by blastomeres from random positions, they contribute to all embryonic lineages. Thus, although four-cell stage blastomeres all have full developmental potential, they differ in their developmental properties.