Male mammals produce spermatozoa throughout adult life from spermatogonial stem cells (SCCs). Like other adult stem cells, the self-renewal and differentiation of SSCs are supported by a `niche', but how does this special microenvironment control these essential SSC functions? Partly, suggest Ralph Brinster and colleagues, by producing colony stimulating factor 1 (Csf1),which acts as an extrinsic stimulator of mouse SSC self-renewal (see p. 1191). The researchers made their discovery by searching for genes that are more highly expressed in the SSC-enriched Thy1+ fraction of mouse pup testes than in the Thy1- fraction. One gene with this pattern of expression encodes the Csf1 receptor. The researchers subsequently found that recombinant Csf1 enhances the self-renewal of mouse SCCs in vitro and that Csf1 expression in mouse testes is localised to Leydig and myoid cells. Together, these results identify Csf1 as a niche factor and suggest that Leydig and myoid cells contribute to SSC niche function in mammals.