Drosophila Echinoid is an

antagonist of Egfr signalling, but is not a member of the L1-type family of cell adhesion molecules Two recent articles published in Development describe the role of the Drosophila Echinoid (Ed) protein in R8 photoreceptor cell development and in Egfr signalling (Rawlins et al., 2003; Spencer and Cagan, 2003). Bai et al. first described Ed as an Ig-domain membrane protein and as an antagonist of Egfr signalling in the developing Drosophila eye (Bai et al., 2001). Both recent publications, which were highlighted in the ‘In this issue ’ feature, speculate that Ed might be an L1-type cell adhesion molecule (CAM). Similar to L1-type proteins, Ed is a homophilic CAM (Islam et al., 2003), and both proteins are members of the Ig-domain superfamily. However, Ed is not part of the L1 family and has a different protein domain structure (usually six Ig- and five FNIII-protein domains for L1-type proteins versus seven plus two for Ed). Furthermore, Ed, as well as its Drosophila paralogue Fred (Chandra et al., 2003), has a strikingly different cytoplasmic domain and lacks the landmark ankyrin binding site of L1-type CAMs. The Drosophila genome contains only one L1-type gene, called Neuroglian (Nrg) (Hortsch, 2000). Interestingly, the Nrg protein is a heterophilic ligand of Ed and triggers the Egfr antagonist activity of Ed in a synergistic manner (Islam et al.