The specification of the R7 photoreceptor cell in each ommatidium of the developing Drosophila eye is dependent on activation of Sevenless receptor tyrosine kinase, 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).