The specification of the R7 photoreceptor cell in each ommatidium of the developing Drosophila eye is dependent on activation of Sevenless receptor tyrosine kinase (sev), which acts via the canonical Ras/Raf/MAP kinase cascade to promote the expression of lz and pros. sev, expressed in presumptive R7 cells, is activated by binding to Bride of Sevenless (boss), a seven-transmembrane protein expressed in R8 cells. (Adapted from FBrf0127283 and FBrf0221727).