The Hox family of transcriptional regulators plays a central role in specifying segment identity along the anteroposterior axis of animal bodies. The Drosophila Hox gene Ultrabithorax (Ubx) is dynamically expressed during development and controls the development of posterior thoracic and anterior abdominal segments. On p. 2951, Claudio Alonso and colleagues show that during development the Ubx gene produces multiple transcripts that vary in their visibility to microRNAs (miRNAs). They demonstrate that different parts of the embryo express Ubx transcripts that contain variable 3′UTRs, each harbouring a distinct set of miRNA target sites. The differential distribution of these transcripts during development is independent of miRNA-mediated degradation but is instead due to an in-built system that processes mRNAs according to developmental context. They also show that other Hox genes, such as Antennapedia, abdominal-A and Abdominal-B, exhibit similar developmental RNA processing and propose that developmental processing of 3′UTR sequences is a general molecular strategy that allows spatiotemporal control of mRNA-miRNA interactions during development.