velopment and sex determinations has become ap- 
parent. The two developmental pathways share a 
number of key regulatory genes: scute, daughter- 
less, deadpan, and extramacrochaete. This pro- 
vides yet another example that a set of genes can be 
considered as a functional cassette, which carries 
out analogous function in different developmental 
contexts. 
Neuronal-type Selector Gene(s) 
Neuronal-type selector genes control the type of 
sensory neuron to which a precursor will give rise. 
For example, the cut locus is required for es organs 
to acquire their correct identity. In the absence of 
cut function, the es organs are transformed into ch 
organs. Recent experiments by Dr. Karen Bloch- 
linger suggest that cut functions as a binary switch. 
If cut is misexpressed in cells that are normally ch 
precursors, they will develop into es organ-like 
structures. This suggests that expression of the cut 
gene activity determines whether the precursor will 
develop into an es or ch organ. The cut product 
contains a homeodomain and is likely to act as a 
transcription factor regulating the expression of 
downstream differentiation genes. 
Dr. Yuh Nung Jan is also Professor of Physiol- 
ogy and of Biochemistry at the University of Cali- 
fornia, San Francisco. 
Articles 
Baldwin, T.J., Tsaur, M.L., Lopez, G.A., Jan, Y.N., 
and Jan, L.Y. 1991. Characterization of a mam- 
malian cDNA for an inactivating voltage-sensitive 
channel. Neuron 7:471-483. 
Bellen, H.J., Vaessin, H., Bier, E., Kolodkin, A., 
D'Evelyn, D., Kooyer, S., and Jan, Y.N. 1992. 
The Drosophila couch potato gene: an essential 
gene required for normal adult behavior. Genet- 
ics 131:365-375. 
Boulianne, G.L., de la Concha, A., Campos-Ortega, 
J .A. , Jan, L.Y. , and Jan, Y.N. 1 99 1 . The Drosoph- 
ila neurogenic gene neuralized encodes a novel 
protein and is expressed in precursors of larval 
and aduh neurons. EMBO J 10:2975-29^5. 
Hartenstein, V., and Jan, Y.N. 1992. Studying Dro- 
sophila embryogenesis with P-lacZ enhancer trap 
lines. Roux's Arch Dev Biol 201 : 194-220. 
Isacoff, E.Y., Jan, Y.N., and Jan, L.Y. 1991. Puta- 
tive receptor for the cytoplasmic inactivation gate 
in the Shaker channel. Nature 353:86-90. 
Jan, L.Y., and Jan, Y.N. 1992. Structural elements 
involved in specific channel functions. Annu 
Rev Physiol 54:537-555. 
Jan, L.Y., and Jan, Y.N. 1992. Tracing the roots of 
ion channels. Ce// 69:71 5-718. 
Jan, Y.N., and Jan, L.Y. 1992. Neuronal specifica- 
tion. Curr Opin Genet Dev 2:608-613. 
Jongens, T.A., Hay, B., Jan, L.Y., and Jan, Y.N. 
1 992. The germ cell-less gene product: a posteri- 
orly localized component necessary for germ cell 
development in Drosophila. Cell 70.569-584. 
Li, M., Jan, Y.N., and Jan, L.Y. 1992. Specification 
of subunit assembly by the hydrophilic amino- 
terminal domain of the Shaker potassium channel. 
Science 257:1225-1230. 
Rao, Y., Vaessin, H., Jan, L.Y. , and Jan, Y.N. 1991. 
Neuroectoderm in Drosophila embryos is depen- 
dent on the mesoderm for positioning but not for 
formation. Genes Dev 5:1577-1588. 
Rutledge, B.J., Zhang, K., Bier, E., Jan, Y.N., and 
Perrimon, N. 1992. The Drosophila spitz gene 
encodes a putative EGF-like growth-factor in- 
volved in dorsal-ventral axis formation and neuro- 
genesis. Genes Dev 6:1505-1517 . 
Sheng, M., Tsaur, M.L., Jan, Y.N., and Jan, L.Y. 
1992. Subcellular segregation of two A-type K"^ 
channel proteins in rat central neurons. Neuron 
9:271-284. 
Timpe, L.C., Isacoff, E., Kimmerly, W., Papa- 
zian, D., Jan, Y.N., and Jan, L.Y. 1991. Molecu- 
lar studies of voltage-gated potassium channels. 
Fidia Res Found Symp Ser 7:9-17. 
Tsaur, M.L., Sheng, M., Lowenstein, D.H., Jan, 
Y.N., and Jan, L.Y. 1992. Differential expression 
of channel mRNAs in the rat brain and down- 
regulation in the hippocampus following sei- 
zures. Neuron 8:1055-1067. 
Vaessin, H , Grell, E., Wolff, E., Bier, E., Jan, L.Y., 
and Jan, Y.N. 1991. prospero is expressed in neu- 
ronal precursors and encodes a nuclear protein 
that is involved in the control of axonal out- 
growth in Drosophila. Cell 67:941-953. 
NEUROSCIENCE 413 
