Using single-cell RNA sequencing, researchers have analysed global gene expression in mouse embryos at multiple defined stages of development. In support of the 'asymmetric hypothesis', Biase et al. found that the individual cells in two-cell and four-cell embryos are not equivalent and that non-trivial, reproducible differences between them influence cell fate decisions. The researchers identified several protein-coding genes with bimodal expression in cells from the same embryo, including some encoding proteins in the WNT signalling pathway, which has a role in cell–cell communication. In a second study, Huang et al. support these findings using a bioinformatic approach: a model for time-variant clustering was used to analyse single-cell gene expression data from mouse pre-implantation embryos. In contrast to the 'equivalence hypothesis', which states that there are no quantitative differences between individual cells of an embryo before the 8-cell stage, the computational analysis suggests that decisions on cell lineage are made as early as the 4-cell stage in mice.