Replicative senescence limits the proliferative potential of cells and protects them against oncogenic signals. In mammalian cells, the proteins encoded by the INK4a-ARF locus play central roles in senescence. INK4a is a cyclin-dependent kinase (CDK) inhibitor whereas ARF stabilizes the tumour suppressor p53. Gordon Peters and colleagues have found that chicken cells do not encode INK4a and now report that the adjacent and closely related INK4b gene instead adopts the central role in senescence in these cells (see p. 2435). The authors show that INK4b mRNA and protein accumulate in senescent chicken embryo fibroblasts (CEFs) and that INK4b is transcriptionally silenced in two immortal chicken cell lines whereas ARF expression is unaffected. The authors also show that knocking down INK4b or ARF by RNAi in CEFs produces only a modest increase in life span - additional factors must therefore contribute to senescence. These observations thus underscore the importance of the INK4b-ARF-INK4a locus in senescence but also reveal that the relative contribution of each gene product varies between species.