Sevenless (sev) is a gene in the fruit fly Drosophila melanogaster that encodes a receptor tyrosine kinase protein essential to the development of the R7 photoreceptor cells in the Drosophila embryonic eye. ^[1] The Drosophila ommatidium contains 8 distinct retinula or R cells, each of which has a different spectral sensitivity. The R7 photo receptor, located in each of several ommatidia in the fly's compound eye, is used to detect ultraviolet light. ^[2] The R8 photoreceptor contains an activator of the RTK (receptor tyrosine kinase) for on a precursor R7 cell, called the bride of sevenless (BOSS). The binding of BOSS to sevenless stimulates a complex series of reactions involving the RTK (sevenless), MAP kinases, Ras and many more molecules to differentiate that precursor R7 photo receptor to a fully functional R7 photo receptor that can see UV light. Much of this knowledge was gained by examining flies with a mutant sevenless which still produced a fully functional R7 photoreceptor when a dominant Ras was injected into the mutant R7 precursor.

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