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., 2003).

Bai, J., Chiu, W., Wang, J., Tzeng, T., Perrimon, N. and Hsu,J. (
2001
). The cell adhesion molecule Echinoid defines a new pathway that antagonizes the Drosophila EGF receptor signaling pathway.
Development
128
,
591
-601.
Chandra, S., Ahmed, A. and Vaessin, H. (
2003
). The Drosophila IgC2 domain protein friend-of-echinoid, a paralogue of echinoid, limits the number of sensory organ precursors in the wing disc and interacts with the Notch signaling pathway.
Dev. Biol.
256
,
302
-316.
Hortsch, M. (
2000
). Structural and functional evolution of the L1-family: are four adhesion molecules better than one?
Mol. Cell. Neurosci.
15
,
1
-10.
Islam, R., Wei, S.-Y., Chiu, W.-H., Hortsch, M. and Hsu,J.-C. (
2003
). Neuroglian activates Echinoid to antagonize the Drosophila EGF receptor signaling pathway.
Development
130
,
2051
-2059.
Rawlins, E. L., White, N. M. and Jarman, A. P.(
2003
). Echinoid limits R8 photoreceptor specification by inhibiting inappropriate EGF receptor signalling within R8 equivalence groups.
Development
130
,
3715
-3724.
Spencer, S. A. and Cagan, R. L. (
2003
). Echinoid is essential for regulation of Egfr signaling and R8 formation during Drosophila eye development.
Development
130
,
3725
-3733.