klingon is a member of the Immunoglobulin superfamily and is expressed in a restricted pattern of neurons during embryonic neurogenesis and in the R7 photoreceptor precursor throughout its development. Starting from the H214 enhancer trap line, we identified a transcription unit, klingon, that encodes a putative protein of 528 amino acids and contains three C2-type Immunoglobulin-like domains followed by one fibronectin type III repeat. When Klingon is expressed in S2 tissue culture cells, it is associated with the cell membrane by a glycosyl-phosphatidylinositol linkage and can mediate homophilic adhesion. Genetic analysis has revealed that klingon is an essential gene that participates in the development of the R7 neuron. Ectopic expression of klingon in all neurons in a sevenless background can alter the position of the R8 rhabdomere.