In yeast, mothers and daughters are genetically identical — they refer to the products of mitotic cell division. But there are also some important differences, which makes yeast a great system for understanding how cell division can give rise to different cell fates — an important general phenomenon. By studying this process in Saccharomyces cerevisiae, Colman–Lerner et al. have found eight genes that are expressed only in daughter cells, and they speculate that the regulation of these genes might constitute a new checkpoint, at the very end of the cell cycle.

Cell division in S. cerevisiae begins when a cell produces a small bud, which grows until the cell undergoes mitosis, and ends when the newly formed daughter cell separates from the mother cell. A well-known difference between mother and daughter cells is that mothers retain a 'bud scar', made of residual cell wall material, whereas daughters have a less obvious 'birth scar'. This is thought to reflect the way the cell wall separates in the final stages of cell division. One of the genes known to be involved in this process is CTS1 , which encodes a cell-wall-degrading enzyme.

Colman–Lerner and colleagues began their study by looking at CTS1 transcription and found that it is expressed only in daughter cells. To see if there might be other genes that are transcribed only in daughters, they analysed published microarray data and found 26 genes that are co-expressed with CTS1 under a variety of conditions. They found that seven of these genes are indeed daughter specific — a neat demonstration of how microarray data can be applied to problems for which the microarray experiment was not originally designed.

Of the eight daughter-specific genes (including CTS1), four are implicated in cell-wall degradation, which suggests that the cell wall is degraded mainly from the daughter's side and might explain why a prominent bud scar forms only on the mother cell. Regulating cell-wall degradation in this way might help to ensure that the cell wall that connects the mother and bud does not break down until the underlying cell membrane has fully separated — a cytokinesis checkpoint. Most significantly, the authors build up a model for the regulation of the daughter-specific genes in which all eight genes are regulated by a trio of proteins — the transcriptional regulator Ace2, the protein kinase Cbk1 and its binding partner Mob2. What's more, all three genes are conserved in other organisms, so Colman–Lerner et al. might well be on the trail of a widespread mechanism for asymmetric cell division.