The Notch signalling pathway regulates cell fate choices during both vertebrate and invertebrate development. In the Drosophila wing disc, the activation of Notch by its ligands Delta and Serrate is required to make the dorsoventral boundary, where several genes, such as wingless and cut, are expressed in a 2- to 4-cell-wide domain. The interactions between Notch and its ligands are modulated by Fringe via a mechanism that may involve post-transcriptional modifications of Notch. The ligands themselves also help to restrict Notch activity to the dorsoventral boundary cells, because they antagonise the activation of the receptor in the cells where their expression is high. This function of the ligands is critical to establish the polarity of signalling, but very little is known about the mechanisms involved in the interactions between Notch and its ligands that result in suppression of Notch activity. The extracellular domain of Notch contains an array of 36 EGF repeats, two of which, repeats 11 and 12, are necessary for direct interactions between Notch with Delta and Serrate. We investigate here the function of a region of the Notch extracellular domain where several missense mutations, called Abruptex, are localised. These Notch alleles are characterised by phenotypes opposite to the loss of Notch function and also by complex complementation patterns. We find that, in Abruptex mutant discs, only the negative effects of the ligands and Fringe are affected, resulting in the failure to restrict the expression of cut and wingless to the dorsoventral boundary. We suggest that Abruptex alleles identify a domain in the Notch protein that mediates the interactions between Notch, its ligands and Fringe that result in suppression of Notch activity.