Early in development, uncommitted mesodermal cells are patterned into repetitive segments and allocated to specific tissue fates - for example,somatic or gut muscles - depending on their position within the segments. In Drosophila, mesodermal subdivision is controlled by levels of the bHLH transcription factor Twist, and, on p. 2359,Tapanes-Castillo and Baylies report that Notch signalling regulates twist expression through a complex network of transcriptional regulators. They show that Notch signalling, mediated by Suppressor of Hairless, not only directly represses twist expression during subdivision of mesodermal segments, but also indirectly regulates twist by activating repressors of twist. The researchers propose that proteins encoded by the Enhancer of split complex and the HLH gene extra machrochaetae are the repressors of twist, and suggest that a similar Notch signalling mechanism may act in mesodermal differentiation in vertebrates.

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