The Orange domain is found in the Drosophila proteins Hesr-1, Hairy, and Enhancer of Split [1,2]. The Orange domain is proposed to mediate specific protein-protein interaction between Hairy and Scute[2].

1: Identification and expression of a novel family of bHLH cDNAs related to Drosophila hairy and enhancer of split. Kokubo H, Lun Y, Johnson RL; Biochem Biophys Res Commun 1999;260:459-465. PMID:10403790

2: Specificity for the hairy/enhancer of split basic helix-loop-helix (bHLH) proteins maps outside the bHLH domain and suggests two separable modes of transcriptional repression. Dawson SR, Turner DL, Weintraub H, Parkhurst SM; Mol Cell Biol 1995;15:6923-6931. PMID:8524259