Nuclear factors in sevenless signalling.

Recent studies in both vertebrates and invertebrates support an 'hourglass' model for signal transduction from receptor tyrosine kinases: Ras channels signals from diverse receptor tyrosine kinases into a common cytoplasmic kinase cascade, the targets of which are an even more diverse collection of nuclear proteins. What are these nuclear factors, and how do they interact to direct specific cellular responses to a generic signal? The past year has brought considerable progress in our quest to answer these questions in one model genetic system, the Drosophila eye.