identification of the appropriate clone, in conjunc- 
tion with P element rescue using cosmid DNA-bear- 
ing P element ends (from a library provided by Dr. 
John Tamkun). Ultimately, rescue of all phenotypes 
by a single transcript should confirm the cosegrega- 
tion analysis. 
Work has also begun on the development of a 
paradigm to study neural target recognition in the 
visual system oi Drosophila. Starting vi'ith a Y maze 
that requires flies to choose between wavelengths 
of light, flies with disrupted vision can be selected; 
this screen was successfully used by Dr. Seymour 
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Benzer many years ago and has been used regularly 
since then by other workers. Among this group of 
mutant flies will be those unable to make connec- 
tions in the optic lobe. These connections can be 
clearly visualized with a whole-mount backfill tech- 
nique developed in the laboratory. In preparation 
for a mutant screen, isogenic stocks with strong 
visual behavior are under construction. 
Dr. Katz is also Assistant Professor of Biochemis- 
try at the University of Texas Southwestern Medical 
Center at Dallas. 
Article 
Katz, R, Moats, W, and Jan, Y.N. 1988. A carbohydrate epitope expressed uniquely on the cell surface Dro- 
sophila neurons is altered in the mutant nac (neurally altered carbohydrate). EMBO J 7:3471-3477. 
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