The complexity of organisms is not simply determined by the number of their genes, but to a large extent by how gene expression is controlled. In addition to transcriptional regulation, this involves several layers of post-transcriptional control, such as translational repression, microRNA-mediated mRNA degradation and translational inhibition, alternative splicing, and the regulated generation of functionally distinct gene products from a single mRNA through alternative use of translation initiation sites. Much progress has been made in describing the molecular basis for these gene regulatory mechanisms. However, it is now a major challenge to translate this knowledge into deeper understanding of the physiological processes, both normal and pathological, that they govern. Using the C/EBP family of transcription factors as an example, the present review describes recent genetic experiments addressing this general problem and discusses how the physiological importance of newly discovered regulatory mechanisms might be determined.