Runx transcription factors, the DNA binding of which is enhanced by CBFβ, play important roles during development and may act as tumour suppressors and oncogenes. Disentangling what Runx proteins do in mammalian systems, where there are three Runx genes, has proved difficult, so researchers have turned to C. elegans, which has only one Runx homologue rnt-1. On p. 3905, Kagoshima and colleagues report that BRO-1, the worm CBFβ homologue, interacts with RNT-1 to promote self-renewal and proliferation of stem cells. They show that bro-1 and rnt-1 deletion mutants both lack some male-specific sensory rays because of failed cell divisions in the stem-cell-like seam cells from which the rays develop. BRO-1 increases the affinity and specificity of RNT-1-DNA interactions, they report, but can also act independently of RNT-1. Finally, co-overexpression of rnt-1 and bro-1 causes seam cell hyperplasia (effectively, a tumour). These new insights into the function of Runx/CBFβ in stem cells support the idea that this DNA-binding complex has oncogenic potential.